BIOLOGY 
LIBRARY 

G 


THE  ANIMAL  BEHAVIOR  SERIES.     VOLUME  II 


THE  ANIMAL   MIND' 


A  Text-book  of  Comparative 
Psychology 


BY 


MARGARET   FLOY  WASHBURN,   Pn.D. 

i»  *  » 

ASSOCIATE  PROFESSOR  OF  PHILOSOPHY 
IN  VASSAR   COLLEGE 


THE   MACMILLAN   COMPANY 
1908 

All  rights  reserved 


W<3 

BIOLOGY 


COPYRIGHT,  1908, 
BY  THE  MACMILLAJSf  COMPANY. 


Set  up  amd  «lectrotyped.     Publish«d  February,  1908. 


Nortoooti 

J.  8.  Gushing  Co.  —  Berwick  &  Smith  Co. 
Norwood,  Mass.,  U.S.A. 


PREFACE 

THE  title  of  this  book  might  more  appropriately,  if  not  more 
concisely,  have  been  "The  Animal  Mind  as  Deduced  from 
Experimental  Evidence."  For  the  facts  set  forth  in  the  fol- 
lowing pages  are  very  largely  the  results  of  the  experimental 
method  in  comparative  psychology.  Thus  many  aspects 
of  the  animal  mind,  to  the  investigation  of  which  experiment 
either  has  not  yet  been  applied  or  is  perhaps  not  adapted,  are 
left  wholly  unconsidered.  This  limitation  of  the  scope  of 
the  book  is  a  consequence  of  its  aim  to  supply  what  I  have 
felt  to  be  a  chief  need  of  comparative  psychology  at  the 
present  time.  Although  the  science  is  still  in  its  formative 
stage,  the  mass  of  experimental  material  that  has  been  accu- 
mulating from  the  researches  of  physiologists  and  psycholo- 
gists is  already  great,  and  is  also  for  the  most  part  inaccessible 
to  the  ordinary  student,  being  widely  scattered  and  to  a  con- 
siderable extent  published  in  journals  which  the  average 
college  library  does  not  contain.  While  we  have  books  on 
animal  instincts  and  on  the  interpretation  of  animal  behavior, 
we  have  no  book  which  adequately  presents  the  simple  facts. 

Probably  no  bibliography  seems  to  one  who  carefully 
examines  it  entirely  consistent  in  what  it  includes  and  what 
it  excludes.  Certainly  the  one  upon  which  this  book  is 
based  contains  inconsistencies.  The  design  has  been  to  ex- 
clude works  bearing  only  upon  general  physiology,  upon  the 
morphology  of  the  nervous  system  and  sense  organs,  or  upon 
the  nature  of  animal  instinct  as  such,  and  to  include  those 
which  bear  upon  the  topics  mentioned  in  the  chapter  headings. 

v 

•       177746 


vi  Preface 

Within  these  limits,  the  collection  of  references  upon  no  topic 
is  as  full  as  would  be  necessary  for  the  bibliography  of  a  spe- 
cial research  upon  that  topic.  Doubtless  there  are  omissions 
for  which  no  excuse  can  be  found.  In  one  or  two  cases,  where 
the  literature  upon  a  single  point  is  very  large,  as  for  example, 
in  the  case  of  the  function  of  the  semicircular  canals,  only  a 
few  of  the  more  important  references  have  been  given. 

One  further  comment  may  be  made.  The  book  through- 
out deals  with  comparative  rather  than  with  genetic  psy- 
chology. 

I  gratefully  acknowledge  help  from  a  number  of  sources. 
To  Professor  Titchener  I  owe,  not  only  my  share  of  that 
genuine  psychological  spirit  which  he  so  successfully  imparts 
to  his  pupils  according  to  their  ability,  but  various  helpful 
criticisms  upon  the  present  work,  about  half  of  which  he 
has  read  in  manuscript.  Dr.  Yerkes  has  given  me  much 
invaluable  aid  in  securing  access  to  material,  and  has  very 
kindly  permitted  me  to  see  the  proofs  of  his  book  on  "The 
Dancing  Mouse."  As  editor  of  the  series  he  has  reviewed 
my  manuscript  to  its  great  advantage.  Professors  Georges 
Bohn  and  George  H.  Parker  have  showed  especial  courtesy 
in  making  their  work  accessible  to  me.  Professor  Jennings 
has  kindly  allowed  the  use  of  a  number  of  illustrations  from 
his  book  on  "The  Behavior  of  the  Lower  Organisms."  My 
colleague  Professor  Aaron  L.  Tread  well  has  generously 
helped  me  in  ways  too  numerous  to  specify.  But  perhaps 
my  heaviest  single  obligation  is  to  Professor  I.  Madison  Bent- 
ley,  who  has  read  the  manuscript  of  the  entire  book,  and 
whose  advice  and  criticism  have  been  of  the  utmost  benefit  to 

every  part  of  it. 

M.  F.  W. 

VASSAR  COLLEGE,  POUGHKEEPSIE,  N.Y. 
October  i,  1907. 


TABLE   OF  CONTENTS 


CHAPTER   I 

THE  DIFFICULTIES  AND  METHODS  OF  COMPARATIVE  PSY- 
CHOLOGY 

PAGES 

§  i.  Difficulties.  §  2.  Methods  of  obtaining  Facts: 
Anecdote.  §3.  Methods  of  obtaining  Facts:  Experi- 
ment. §  4.  Methods  of  Qbtaining  Facts :  the  Ideal 
Method.  §5.  Methods  of  Interpreting  Facts  .  .  1-26 

CHAPTER   II 

THE  EVIDENCE  OF  MIND 

§6.  Inferring  Mind  from  Behavior.     §7.  Inferring  Mind 

from  Structure 27-36 

CHAPTER  III 

THE  MIND  OF  THE  SIMPLEST  ANIMALS 

§  8.  The  Structure  and  Behavior  of  Amreba.  §  9.  The 
Mind  of  Amoeba.  §  10.  The  Structure  and  Behavior 
of  Paramecium.  §  11.  The  Mind  of  Paramecium. 
§  12.  Definitions  of  Tropisms 37~S7 

CHAPTER  IV 

SENSORY  DISCRIMINATION:  METHODS  OF  INVESTIGATION 

§  13.  Preliminary  Considerations.  §  14.  Structure  as 
Evidence  of  Discrimination.  §  15.  Behavior  as  Evi- 
dence of  Discrimination.  §  16.  Evidence  from  Structure 
and  Behavior  Combined.  §  17.  Evidence  for  Discrimi- 
nation of  Certain  "  Lower  "  Sensation  Classes  .  .  58-66 

vii 


viii  Table  of  Contents 

CHAPTER  V 
SENSORY  DISCRIMINATION:  THE  CHEMICAL  SENSE 

PAGES 

§  18.  The  Chemical  Sense  in  Ccelenterates.  §  19.  The 
Chemical  Sense  in  Flatworms.  §  20.  The  Chemical 
Sense  in  Annelids.  §  21.  The  Chemical  Sense  in  Mol- 
lusks.  §  22.  The  Chemical  Sense  in  Echinoderms. 
§  23.  The  Chemical  Sense  in  Crustacea.  §  24.  The 
Chemical  Sense  in  Arachnida.  §  25.  The  Chemical 
Sense  in  Insects.  §  26.  How  Ants  find  Food.  §  27.  How 
Ants  find  the  Way  Home.  §  28.  How  Ants  "  recognize  " 
Nest  Mates.  §  29.  How  Bees  are  attracted  to  Flowers. 
§  30.  How  Bees  find  the  Hive.  §  31.  How  Bees  "rec- 
ognize" Nest  Mates.  §  32.  The  Chemical  Sense  in 
Vertebrates 67-105 


CHAPTER  VI 

SENSORY  DISCRIMINATION:  HEARING 

§  33.  Hearing  in  Lower  Invertebrates.  §  34.  Hearing 
in  Crustacea.  §  35.  Hearing  in  Spiders.  §  36.  Hear- 
ing in  Insects.  §  37.  Hearing  in  Fishes.  §  38.  Hear- 
ing in  Amphibia.  §39.  Hearing  in  Higher  Vertebrates  106-119 


CHAPTER  VII 

SENSORY  DISCRIMINATION:  VISION 

§  40.  Some  Problems  connected  with  Vision.  §  41.  Vis- 
ion in  Protozoa.  §  42.  Vision  in  Coelenterates. 
§  43.  Vision  in  Planarians.  §  44.  Vision  in  Annelids. 
§  45.  Vision  in  Mollusks.  §  46.  Vision  in  Echino- 
derms. §  47.  Vision  in  Crustacea.  §  48.  Vision  in 
Spiders.  §  49.  Vision  in  Insects.  §  50.  Vision  in 
Amphioxus  and  in  Fish.  §  51.  Vision  in  Amphibia. 
§  52.  Vision  in  Other  Vertebrates  ....  120-147 


Table  of  Contents 


IX 


CHAPTER  VIII 


SPATIALLY  DETERMINED  REACTIONS  AND  SPACE  PERCEP- 
TION 

§  53.  Classes  of  Spatially  Determined  Reactions. 
§  54.  Class  I :  Reactions  to  a  Single  Localized  Stimu- 
lus. §  55.  Class  II:  Orienting  Reactions:  Possible 
Modes  of  Producing  Them.  §  56.  Orientation  to  Gravity : 
Protozoa.  §  57.  Orientation  to  Gravity :  Coelenterates. 
§58.  Orientation  to  Gravity :  Planarians.  §59.  Orien- 
tation to  Gravity:  Annelids.  §  60.  Orientation  to 
Gravity:  Mollusks.  §  61.  Orientation  to  Gravity: 
Echinoderms.  §62.  Orientation  to  Gravity :  Crustacea. 
§  63.  Orientation  to  Gravity:  Spiders  and  Insects. 
§64.  Orientation  to  Gravity :  Vertebrates.  §65.  The 
Psychic  Aspect  of  Orientation  to  Gravity.  §  66.  Orien- 
tation to  Light :  Photopathy  and  Phototaxis.  §  67.  In- 
stances of  Photopathy  and  Phototaxis.  §  68.  Direction 
and  Intensity  Theories  of  Phototaxis.  §  69.  The  Eyes 
in  Phototaxis.  §  70.  Influences  affecting  the  Sense  of 
Light  Orientations.  §  71.  Mutual  Influence  of  Light 
and  Gravity  Orientations.  §  72.  The  Psychic  Aspect 
of  Orientation  to  Light.  §  73.  Orientation  to  Other 
Forces  . 


148-189 


CHAPTER  IX 

SPATIALLY  DETERMINED  REACTIONS  AND  SPACE  PERCEP- 
TION (continued) 

§  74.  Class  III :  Reaction  to  a  Moving  Stimulus. 
§75.  Class  IV:  Reaction  to  an  Image.  §76.  Methods 
of  Investigating  the  Visual  Image:  the  Size  Test. 
§  77.  Methods  of  Investigating  the  Visual  Image :  the 
Form  Test.  §  78.  Class  V :  Reactions  adapted  to  the 
Distance  of  Objects.  §  79.  Some  Theoretical  Consid- 
erations 


190-204 


Table  of  Contents 


CHAPTER  X 

THE  MODIFICATION  OF   CONSCIOUS  PROCESSES  BY  INDI- 
VIDUAL EXPERIENCE 

PAGES 

§80.  Absence  of  Modification.  §  81.  Heightened  Reac- 
tion as  the  Result  of  Previous  Stimulation.  §  82.  Ces- 
sation of  Reaction  to  a  Repeated  Stimulus.  §  83.  Varied 
Negative  Reaction  to  a  Repeated  Stimulus.  §  84.  Drop- 
ping off  Useless  Movements :  the  Labyrinth  Method. 
§  85.  Dropping  off  Useless  Movements  :  the  Puzzle-box 
Method.  §  86.  The  Psychic  Aspect  of  Dropping  off 
Useless  Movements 205-246 

CHAPTER  XI 

THE    MODIFICATION  OF   CONSCIOUS  PROCESSES  BY  INDI- 
VIDUAL EXPERIENCE  (continued} 

§  87.  The  Inhibition  of  Instinct.  §  88.  Inhibition  in- 
volving Discrimination  of  Successive  Stimuli.  §  89.  In- 
hibition involving  Discrimination  of  Simultaneous 
Stimuli.  §90.  Comparison  of  Methods.  §91.  "Visual 
Memory "  in  Homing 247-269 

CHAPTER  XII 
THE  MEMORY  IDEA 

(m  • 

§  92.  Evidence  for  and  against  Ideas  in  Animals. 
§  93.  The  Primitive  Function  of  Ideas.  §  94.  The  ^ 

Significance  of  Stimuli  from  a  Distance.  §  95.  Ideas  of 
Movement 270-284 

*s 

CHAPTER  XIII  "\\ 

SOME  ASPECTS  OF  ATTENTION 

§  96.  The  Interference  of  Stimuli.  §  97.  Methods  of 
Securing  Prepotency  of  Vitally  Important  Stimuli. 
§  98.  The  Peculiar  Characteristics  of  Attention  as  a 
Device  to  secure  Prepotency 285-294 


THE   ANIMAL   MIND 


THE  ANIMAL  MIND 


CHAPTER  I 

THE  DIFFICULTIES  AND  METHODS  OF  COMPARATIVE 
PSYCHOLOGY 

§  i.   Difficulties 

[THAT  the  mind  of  each  human  being  forms  a  region  inac- 
cessible to  all  save  its  possessor,  is  one  of  the  commonplaces 
of  reflection.  His  neighbor's  knowledge  of  each  person's 
mind  must  always  be  indirect,  a  matter  of  inference. \  How 
wide  of  the  truth  this  inference  may  be,  even  under  the  most 
favorable  circumstances,  is  also  an  affair  of  everyday  ex- 
perience: each  of  us  can  judge  his  fellow-men  only  on  the 
basis  of  his  own  thoughts  and  feelings  in  similar  circum- 
stances, and  the  individual  peculiarities  of  different  members 
of  the  human  species  are  of  necessity  very  imperfectly  com- 
prehended by  others.  The  science  of  human  psychology 
has  to  reckon  with  this  unbridgable  gap  between  minds  as 
its  chief  difficulty.  The  psychologist  may  look  into  his  own 
mind  and  study  its  workings  with  impartial  insight,  yet  he 
can  never  be  sure  that  the  laws  which  he  derives  from  such 
a  study  are  not  distorted  by  some  personal  twist  or  bias.  For 
example,  it  has  been  suggested  that  the  philosopher  Hume 
was  influenced  by  his  tendency  toward  a  visual  type  of 
imagination  in  his  discussion  of  the  nature  of  ideas,  which 
to  him  were  evidently  visual  images.  As  is  well  known,  the 
experimental  method  in  psychology  has  aimed  to  minimize 


2  The  Animal  Mind 

the  danger  of  confusing  individual  peculiarities  with  general 
mental  laws.  In  a  psychological  experiment,  an  unbiassed 
observer  is  asked  to  study  his  own  experience  under  certain 
definite  conditions,  and  to  put  it  into  words  so  that  the  ex- 
perimenter may  know  what  the  contents  of  another  mind  are 
like  in  the  circumstances.  Thus  language  is  the  essential 
apparatus  in  experimental  psychology;  language  with  all 
its  defects,  its  ambiguity,  its  substitution  of  crystallized  con- 
cepts for  the  protean  flux  of  actually  lived  experience,  its 
lack  of  terms  to  express  those  parts  of  experience  which  are 
of  small  practical  importance  in  everyday  life,  but  which 
may  be  of  the  highest  importance  to  mental  science.  Out- 
side of  the  psychological  laboratory  language  is  not  always 
the  best  guide  to  the  contents  of  other  minds,  because  it  is 
not  always  the  expression  of  a  "genuine  wish  to  communicate 
thought.  *4^diPJi§jy2£ak  louder  thanwords,"  the  proverb 
says ;  but  when  wordTar?l)acked'D^rgoocl  faith  they  furnish 
by  far  the  safest  indication  of  the  thought  of  others.  Whether, 
however,  our  inferences  are  made  on  the  basis  of  words  or 
of  actions,  they  are  all  necessarily  made  on  the  hypothesis 
that  human  minds  are  built  on  the  same  pattern,  that  what 
a  given  word  or  action  would  mean  for  my  mind,  this  it 
means  also  for  my  neighbor's  mind. 

If  this  hypothesis  be  uncertain  when  applied  to  our  fellow 
human  beings,  it  fails  us  utterly  when  we  turn  to  the  lower 
animals.  If  my  neighbor's  mind  is  a  mystery  to  me,  how 
great  is  the  mystery  which  looks  out  of  the  'eyes  of  a  dog,  and 
how  insoluble  the  problem  presented  by  the  mind  of  an 
invertebrate  animal,  an  ant  or  a  spider  !  We  know  that  such 
minds  must  differ  from  ours  not  only  in  certain  individual 
peculiarities,  but  in  ways  at  whose  nature  we  can  only  guess. 
The  nervous  systems  of  many  animals  vary  widely  from  our 
Newn.  We  have,  perhaps,  too  little  knowledge  about  the 


Difficulties  and  Methods  3 

functions  of  our  own  to  conjecture  with  any  certainty  what 
difference  this  must  make  in  the  conscious  life  of  such  animals  ; 
but  when  we  find  sense  organs,  such  as  the  compound  eyes 
of  insects  or  crustaceans,  constructed  on  a  plan  wholly  diverse 
from  that  of  ours ;  when  we  find  organs  apparently  sensory  in 
function,  but  so  unlike  our  own  that  we  cannot  tell  what  pur- 
pose they  serve,  —  we  are  baffled  in  our  attempt  to  construct 
the  mental  life  of  the  animals  possessing  them,  for  lack  of 
power  to  supply  the  sensation  elements  of  that  life.  "It  is 
not,"  said  Locke,  "in  the  power  of  the  most  exalted  wit  or 
enlarged  understanding,  by  any  quickness  or  variety  of 
thought,  to  invent  or  frame  one  new  simple  idea  in  the  mind" 
(232,  Bk.  II,  ch.  2);  we  cannot  imagine  a  color  or  a  sound 
or  a  smell  that  we  have  never  experienced;  how  much  less 
the  sensations  of  a  sense  radically  different  from  any  that  we 
possess !  Again,  a  bodily  structure  entirely  unlike  our  own 
must  create  a  background  of  organic  sensation  which  renders 
the  whole  mental  life  of  an  animal  foreign  and  unfamiliar  to 
us.  We  speak,  for  example,  of  an  "angry"  wasp.  Anger, 
in  our  own  experience,  is  largely  composed  of  sensations  of 
quickened  heart  beat,  of  altered  breathing,  of  muscular  ten- 
sion, of  increased  blood  pressure  in  the  head  and  face.  The 
circulation  of  a  wasp  is  fundamentally  different  from  that  of 
any  vertebrate.  The  wasp  does  not  breathe  through  lungs, 
it  wears  its  skeleton  on  the  outside,  and  it  has  the  muscles 
attached  to  the  inside  of  the  skeleton*.  What  is  anger  like 
in  the  wasp's  consciousness?  W«  c$n  ffrm  n*  atfeQuate 
idea  of  it. 

To  this  fundamental  difficulty  of  the  dissimilarity  between 
animal  minds  and  ours  is  added,  of  course,  the  obstacle  that 
animals  have  no  language  in  which  to  describe  their  expe- 
rience to  us.  Where  this  unlikeness  is  greatest,  as  in  the 
case  of  invertebrate  animals,  language  would  be  of  little  use, 


4  The  Animal  Mind 

since  we  could  not  interpret  it  from  our  experience ;  but  the 
higher  vertebrates  could  give  us  much  insight  into  their 
minds  if  they  could  only  speak.  We  are,  however,  restricted 
to  the  inferences  we  can  draw  from  movements  and  sounds 
that  are  made  for  the  most  part  without  the  intention  of 
communicating  anything  to  us.  One  happy  consequence  of 
this  fact,  which  to  a  slight  extent  balances  its  disadvantages, 
is  that  we  have  not  to  contend  with  self-consciousness  and 
posing,  which  often  invalidate  human  reports  of  introspec- 
tion. 

From  these  general  considerations  we  can  understand 
something  of  the  special  difficulties  that  beset  the  path  of  the 
comparative  psychologist,  who  desires  to  know  the  contents 
of  minds  below  the  human  level.  Knowledge  regarding  the 
animal  mind,  like  knowledge  of  human  minds  other  than  our 
own,  must  come  by  way  of  inference  from  behavior.  Two 
fundamental  questions  then  confront  the  comparative  psy- 
chologist. First,  by  what  method  shall  he  find  out  how  an 
animal  behaves?  Second,  how  shall  he  interpret  the  con- 
scious aspect  of  that  behavior? 

§  2.   Methods  of  Obtaining  Facts:  The  Method  of  Anecdote 

The  reading  of  such  a  book  as  Romanes's  "Animal  Intelli- 
gence, "  or  of  the  letters  about  animal  behavior  in  the  London 
Spectator,  will  reveal  one  method  of  gathering  information 
aboftfdiat  anir^ls  do.  -  This  has  been  termed  the  Method 
of  Anecdote.  It  consists  essentially  in  taking  the  report  of 
another  person  regarding  the  action  of  an  animal,  observed 
most  commonly  by  accident,  and  attracting  attention  because 
of  its  unusual  character.  In  certain  cases  the  observer  while 
engaged  in  some  other  pursuit  happens  to  notice  the  singular 
behavior  of  an  animal,  and  at  his  leisure  writes  out  an  account 


Difficulties  and  Methods  5 

of  it.  In  others,  the  animal  is  a  pet,  in  whose  high  intellectual 
powers  its  master  takes  pride.  It  is  safe  to  say  that  this 
method  of  collecting  information  always  labors  under  at 
least  one,  and  frequently  under  several,  of  the  following  dis- 
advantages :  — 

1.  The  observer  is  not  scientifically  trained  to  distinguish 
what  he  sees  from  what  he  infers. 

2.  He  is  not  intimately  acquainted  with  the  habits  of  the 
species  to  which  the  animal  belongs. 

3.  He  is  not  acquainted  with  the  past  experience  of   the 
individual  animal  concerned. 

4.  He  has  a  'personal  affection  for  the  animal  concerned, 
and  a  desire  to  show  its  superior  intelligence. 

5.  He  has  the  desire,  common  to  all  humanity,  to  tell  a 
good  story. 

Some  of  these  tendencies  to  error  it  is  unnecessary  to  illus- 
trate. A  good  example  of  the  dangers  of  (2),  lack  of  acquaint- 
ance with  the  habits  of  the  species,  is  given  by  Mr.  and  Mrs. 
Peckham.  They  quote  the  following  anecdote  reported  by 
no  less  eminent  and  trained  an  observer  than  Wundt.  "I 
had  made  myself,"  says  that  psychologist,  "as  a  boy,  a  fly- 
trap like  a  pigeon  cote.  The  flies  were  attracted  by  scatter- 
ing sugar  and  caught  as  soon  as  they  had  entered  the  cage. 
Behind  the  trap  was  a  second  box,  separated  from  it  by  a 
sliding  door,  which  could  be  opened  or  shut  at  pleasure.  In 
this  I  had  put  a  large  garden  spider.  Cage  and  box  were 
provided  with  glass  windows  on  the  top,  so  that  I  could  quite 
well  observe  anything  that  was  going  on  inside.  .  .  .  When 
some  flies  had  been  caught,  and  the  slide  was  drawn  out,  the 
spider  of  course  rushed  upon  her  prey  and  devoured  them. 
.  .  .  This  went  on  for  some  time.  The  spider  was  some- 
times let  into  the  cage,  sometimes  confined  to  her  own  box. 
But  one  day  I  made  a  notable  discovery.  During  an  absence 


6  The  Animal  Mind 

the  slide  had  been  accidentally  left  open  for  some  little 
while.  When  I  came  to  shut  it,  I  found  that  there  was  an 
unusual  resistance.  As  I  looked  more  closely,  I  found  that 
the  spider  had  drawn  a  large  number  of  thick  threads  directly 
under  the  lifted  door,  and  that  these  were  preventing  my  clos- 
ing it.  .  .  ." 

"What  was  going  on  in  the  spider's  mind?"  Wundt  asks, 
and  points  out  that  it  is  unnecessary  to  assume  that  she 
understood  and  reasoned  out  the  mechanical  requirements 
of  the  situation.  The  whole  matter  can  be  explained,  he 
thinks,  in  a  simpler  way.  "I  imagine  that  as  the  days  went 
by  there  had  been  formed  in  the  mind  of  the  spider  a  deter- 
minate association  on  the  one  hand  between  free  entry  into 
the  cage  and  the  pleasurable  feeling  attending  satisfaction  of 
the  nutritive  impulse,  and  on  the  other  between  the  closed 
slide  and  the  unpleasant  feeling  of  hunger  and  inhibited  im- 
pulse. Now  in  her  free  life  the  spider  had  always  employed 
her  web  in  the  service  of  the  nutritive  impulse.  Associations 
had  therefore  grown  up  between  the  definite  positions  of  her 
web  and  definite  peculiarities  of  the  objects  to  which  it  was 
attached,  as  well  as  changes  which  it  produced  in  the  positions 
of  certain  of  these  objects,  —  leaves,  small  twigs,  etc.  The 
impression  of  the  falling  slide,  that  is,  called  up  by  association, 
the  idea  of  other  objects  similarly  moved  which  had  been  held 
in  their  places  by  threads  properly  spun;  and  finally  there 
were  connected  with  this  association  the  other  two  of  pleasure 
and  raising,  unpleasantness  and  closing,  of  the  door"  (446, 

PP-  SSJ-IS2)- 

The  Peckhams  remark  in  criticism  of  this  observation: 

"Had  Wundt  been  familiar  with  the  habits  of  spiders,  he 
would  have  known  that  whenever  they  are  confined  they  walk 
around  and  around  the  cage,  leaving  behind  them  lines  of 
web.  Of  course  many  lines  passed  under  his  little  sliding 


Difficulties  and  Methods  7 

door,  and  when  he  came  to  close  it  there  was  a  slight  resist- 
ance. These  are  the  facts.  His  inference  that  there  was  even 
a  remotest  intention  on  the  part  of  his  prisoner  to  hinder  the 
movement  of  the  door  is  entirely  gratuitous.  Even  the 
simpler  mental  states  that  are  supposed  to  have  passed 
through  the  mind  of  the  spider  were  the  products  of  Wundt's 
own  imagination  "  (322,  p.  230).  The  fact  that  the  anecdote 
was  a  recollection  of  childhood,  so  that  it  would  probably  be 
impossible  to  bring  any  evidence  from  the  character  of  the 
web  or  other  circumstance  against  the  suggestion  of  Mr.  and 
Mrs.  Peckham,  is  a  further  instance  of  the  unscientific  use 
of  anecdotal  testimony. 

An  illustration  of  the  third  objection  mentioned  above, 
the  disadvantage  of  ignorance  of  the  animal's  individual  his- 
tory, is  furnished  by  Lloyd  Morgan.  In  describing  his  futile 
efforts  to  teach  a  fox  terrier  the  best  way  to  pull  a  crooked 
stick  through  a  fence,  he  says  that  the  dog  showed  no  sign 
"of  perceiving  that  by  pushing  the  stick  and  freeing  the  crook 
he  could  pull  the  stick  through.  Each  time  the  crook  caught 
he  pulled  with  all  his  strength,  seizing  the  stick  now  at  the 
end,  now  in  the  middle,  and  now  near  the  crook.  At  length 
he  seized  the  crook  itself  and  with  a  wrench  broke  it  off.  A 
man  who  was  passing  .  .  .  said,  'Clever  dog  that,  sir;  he 
knows  where  the  hitch  do  lie.'  The  remark  was  the  charac- 
teristic outcome  of  two  minutes'  chance  observation  "  (282, 
pp.  142-143).  How  many  anecdotes  of  animals  are  based 
on  similar  accidents? 

It  will  be  seen  that  in  both  the  cases  just  criticised  the  error 
lies  in  the  interpretation  of  the  animal's  behavior.  Indeed, 
a  root  of  evil  in  the  method  of  anecdote  consists  in  the  fact 
that  observation  in  this  form  is  imperfectly  divorced  from  in- 
terpretation. The  maker  of  an  anecdote  is  seldom  content 
with  merely  telling  one  what  the  animal  did  and  leaving  future 


8  The  Animal  Mind 

investigation  and  the  comparative  study  of  many  facts  to  de- 
cide what  the  animal's  conscious  experience  in  doing  it  was 
like.  The  point  of  the  anecdote  usually  consists  in  showing 
that  a  human  interpretation  of  the  animal's  behavior  is  pos- 
sible. Here  is  shown  the  desire  to  tell  a  good  story,  which 
we  mentioned  among  the  pitfalls  of  the  anecdotal  method; 
the  wish  to  report  something  unusual,  not  to  get  a  just  concep- 
tion of  the  normal  behavior  of  an  animal.  As  Thorndike  has 
forcibly  put  it :  "Dogs  get  lost  hundreds  of  times  and  no  one 
ever  notices  it  or  sends  an  account  of  it  to  a  scientific  maga- 
zine. But  let  one  find  his  way  from  Brooklyn  to  Yonkers 
and  the  fact  immediately  becomes  a  circulating  anecdote. 
Thousands  of  cats  on  thousands  of  occasions  sit  helplessly 
yowling,  and  no  one  takes  thought  of  it  or  writes  to  his  friend, 
the  professor ;  but  let  one  cat  claw  at  the  knob  of  a  door  sup- 
posedly as  a  signal  to  be  let  out,  and  straightway  this  cat  be- 
comes the  representative  of  the  cat-mind  in  all  the  books  " 

(393,  P-  4). 

All  this  is  not  to  deny  that  much  of  the  testimony  to  be 
found  in  Romanes's  " Animal  Intelligence"  and  Darwin's 
" Descent  of  Man"  is  the  trustworthy  report  of  trained  ob- 
servers ;  but  it  is  difficult  to  separate  the  grain  from  the  chaff, 
and  one  feels  toward  many  of  the  anecdotes  the  attitude  of 
scepticism  produced,  for  example,  by  this  tale  which  an 
Australian  lady  reported  to  the  Linnaean  Society.  The  burial 
of  some  deceased  comrades  was  accomplished,  she  says,  by 
a  nest  of  " soldier  ants"  near  Sydney,  in  the  following  fashion. 
"All  fell  into  rank  walking  regularly  and  slowly  two  by  two, 
until  they  arrived  at  the  spot  where  lay  the  dead  bodies.  .  .  . 
Two  of  the  ants  advanced  and  took  up  the  dead  body  of  one 
of  their  comrades ;  then  two  others,  and  so  on  until  all  were 
ready  to  march.  First  walked  two  ants  bearing  a  body, 
then  two  without  a  burden;  then  two  others  with  another 


Difficulties  and  Methods  9 

dead  ant,  and  so  on,  until  the  line  was  extended  to  about  forty 
pairs,  and  the  procession  now  moved  slowly  onward,  followed 
by  an  irregular  body  of  about  two  hundred  ants.  Occa- 
sionally the  two  laden  ants  stopped,  and  laying  down  the  dead 
ant,  it  was  taken  up  by  the  two  walking  unburdened  behind 
them,  and  thus,  by  occasionally  relieving  each  other,  they 
arrived  at  a  sandy  spot  near  the  sea."  A  separate  grave  was 
then  dug  for  each  dead  ant.  "Some  six  or  seven  of  the  ants 
had  attempted  to  run  off  without  performing  their  share  of 
the  task  of  digging;  these  were  caught  and  brought  back, 
when  they  were  at  once  attacked  by  the  body  of  ants  and 
killed  upon  the  spot.  A  single  grave  was  quickly  dug  and 
they  were  all  dropped  into  it."  No  funeral  procession  for 
them !  Of  this  story  Romanes  says,  "The  observation  seems 
to  have  been  one  about  which  there  could  scarcely  have  been 
a  mistake  "  (364,  p.  91).  One  is  inclined  to  think  it  just 
possible  that  there  was. 

§3.    Methods  of  Obtaining  Facts :  The  Method  of  Experiment 

Diametrically  opposed  to  the  Method  of  Anecdote  and  its 
unscientific  character  is  the  Method  of  Experiment.  An 
experiment,  properly  conducted,  always  implies  that  the 
conditions  are  controlled,  or  at  least  known;  whereas  igno- 
rance of  the  conditions  is,  as  we  have  seen,  a  common  feature 
of  anecdote.  The  experimenter  is  impartial ;  he  has  no  de- 
sire to  bring  about  any  particular  result.  The  teller  of  an 
anecdote  wishes  to  prove  animal  intelligence.  The  experi- 
menter is  willing  to  report  the  facts  precisely  as  he  observes 
them,  and  is  in  no  haste  to  make  them  prove  anything. 
The  conduct  of  an  experiment  upon  an  animal  will,  of  course, 
vary  according  to  the  problem  to  be  solved.  If  the  object  is 
to  test  some  innate  reaction  on  the  animal's  part,  such  as  its 
ordinary  responses  to  stimulation  or  its  instincts,  one  need 


io  The  Animal  Mind 

merely  place  the  animal  under  favorable  conditions  for  ob- 
servation, make  sure  that  it  is  not  frightened  or  in  an  abnormal 
state,  supply  the  appropriate  stimulus  unmixed  with  others, 
and  watch  the  result.  If  it  is  desired  to  study  the  process 
by  which  an  animal  learns  to  adapt  itself  to  a  new  situation, 
one  must,  of  course,  make  sure  in  addition  that  the  situation 
really  is  new  to  the  animal,  and  yet  that  it  makes  sufficient 
appeal  to  some  instinctive  tendency  to  supply  a  motive  for 
the  learning  process. 

As  one  might  expect,  among  the  earliest  experiments  upon 
animals  were  those  made  by  physiologists  with  a  view  to 
determining  the  functions  of  sense  organs.  The  experimental 
movement  in  psychology  was  slow  in  extending  itself  into  the 
field  of  the  animal  mind. 

Romanes,  whose  adherence  to  the  anecdotal  method  we 
have  noted,  made  in  1881,  rather  as  a  physiologist  than  as  a 
psychologist,  a  number  of  exact  and  highly  valued  experi- 
ments on  ccelenterates  and  echinoderms,  which  were  sum- 
marized in  his  book  entitled  "  Jelly-fish,  Star- fish,  and  Sea- 
urchins,"  published  in  1885.  He  has  also  recorded  some 
rather  informal  experiments  on  the  keenness  of  smell  in 
dogs.  Sir  John  Lubbock,  in  1883,  reported  the  results  of 
some  experiments  on  the  color  sense  of  the  small  crustacean 
Daphnia,  and  his  book  on  "Ants,  Bees,  and  Wasps,"  con- 
taining an  account  of  experimental  tests  of  the  senses  and 
"intelligence"  of  these  insects,  appeared  in  the  same  year. 
A  German  entomologist,  Vitus  Graber,  experimented  very 
extensively  at  about  this  period  on  the  senses  of  sight  and 
smell  in  many  animals.  Preyer,  the  authority  on  child 
psychology,  published  in  1886  an  experimental  study  of  the 
behavior  of  the  starfish.  Loeb's  work  on  the  reactions  of 
animals  to  stimulation  began  to  appear  in  1888.  Bethe's 
experiments  on  ants  and  bees  were  published  in  1898.  Max 


Difficulties  and  Methods  n 

Verworn,  the  physiologist,  published  in  1899  an  exhaustive 
experimental  study  of  the  behavior  of  single-celled  animals. 
With  the  exception  of  Preyer  and  Romanes,  all  these  men 
had  but  a  secondary  interest  in  comparative  psychology: 
Bethe,  indeed,  as  we  shall  see,  wholly  rejects  it.  Lloyd 
Morgan,  who  has  written  instructively  on  comparative 
psychology,  makes  but  a  limited  use  of  the  experimental 
method.  Wesley  Mills,  professor  of  physiology  in  Me  Gill 
University,  has  studied  very  carefully  the  mental  develop- 
ment of  young  animals  such  as  cats  and  dogs,  but  is  inclined 
to  criticise  the  use  of  experiment  in  observing  animals.  The 
work  of  E.  L.  Thorndike,  whose  "  Animal  Intelligence " 
appeared  in  1898,  represents,  perhaps,  the  first  definite  effect 
of  the  modern  experimental  movement  in  psychology  upon 
the  study  of  the  animal  mind.  Thorndike's  aim  in  this  re- 
search was  to  place  his  animals  (chicks,  cats,  and  dogs)  under 
the  most  rigidly  controlled1  experimental  conditions.  The 
cats  and  dogs,  reduced  by  fasting  to  a  state  of  "utter  hunger," 
were  placed  in  boxes,  with  food  outside,  and  the  process 
whereby  they  learned  to  work  the  various  mechanisms  which 
let  them  out  was  carefully  observed.  Since  the  appear- 
ance of  Thorndike's  work  the  performance  of  experiments 
upon  animals  has  played  much  part  in  the  work  of  psychologi- 
cal laboratories,  particularly  those  of  Harvard,  Clark,  and 
Chicago  universities.  The  biologists  and  physiologists  have 
continued  their  researches  by  this  method,  so  that  a  very  large 
amount  of  experimental  work  is  now  being  done  in  compara- 
tive psychology. 

Despite  the  obvious  advantages  of  experiment  as  a  method 
for  the  study  of  animal  behavior,  it  is  not  without  its  dangers. 
These  were  clearly  stated  by  Wesley  Mills  in  a  criticism  of 
Thorndike's  " Animal  Intelligence"  (273).  They  may  be 
summed  up  by  saying  that  there  is  a  risk  of  placing  the 


12  The  Animal  Mind 

animal  experimented  upon  under  abnormal  conditions  in  the 
attempt  to  make  them  definite  and  controllable.1  Did  not, 
for  example,  the  extreme  hunger  to  which  Thorndike's  cats 
and  dogs  were  reduced,  while  it  simplified  the  conditions 
in  one  sense  by  making  the  strength  of  the  motive  to  escape 
as  nearly  as  possible  equal  for  all  the  animals,  complicate 
matters  in  another  sense  by  diminishing  their  capacity  to 
learn?  Were  the  animals  perhaps  frightened  and  dis- 
tracted by  the  unusual  character  of  their  surroundings? 
Thorndike  thinks  not  (396);  but  whether  or  no  he  suc- 
ceeded in  averting  these  dangers,  it  is  clear  that  they  are 
real.  It  is  also  obvious  that  they  are  the  more  threatening, 
the  higher  the  animal  with  which  one  has  to  deal.  Fright, 
bewilderment,  loneliness,  are  conditions  more  apt  to  be  met 
with  among  the  higher  vertebrates  than  lower  down  in  the 
scale,  and  the  utmost  care  should  be  taken  to  make  sure  that 
animals  likely  to  be  affected  by  them  are  thoroughly  trained 
and  at  home  in  their  surroundings  before  the  experimenter 
records  results. 

§  4.   Methods  of  Obtaining  Facts:    The  Ideal  Method. 

The  ideal  method  for  the  study  of  a  higher  animal  involves 
patient  observation  upon  a  specimen  known  from  birth, 
watched  in  its  ordinary  behavior  and  environment,  and 
occasionally  experimented  upon  with  proper  control  of  the 
conditions  and  without  frightening  it  or  otherwise  ren- 
dering it  abnormal.  The  observer  should  acquaint  him- 
self with  the  individual  peculiarities  of  each  animal  studied, 
for  there  is  no  doubt  that  striking  differences  in  mental  capac- 
ity occur  among  the  individuals  of  a  single  species.  At  the 
same  time  that  he  obtains  the  confidence  of  each  individual 

1  Cf.  also  Kline  (222),  and  Vaschide  and  Rousseau  (413). 


Difficulties  and  Methods  13 

animal,  he  should  be  able  to  hold  in  check  the  tendency  to 
humanize  it  and  to  take  a  personal  pleasure  in  its  achieve- 
ments if  it  be  unusually  endowed.  This  is,  to  say  the  least, 
not  easy.  Absolute  indifference  to  the  animals  studied,  if 
not  so  dangerous  as  doting  affection,  is  yet  to  be  avoided. 

§  5.    Methods  of  Interpreting  Facts 

We  may  now  turn  from  the  problem  of  discovering  the  facts 
about  animal  behavior  to  the  problem  of  interpreting  them. 
If  an  animal  behaves  in  a  certain  manner,  what  may  we  con- 
clude the  consciousness  accompanying  its  behavior  to  be 
like?  As  we  have  seen,  the  interpretation  is  often  confused 
with  the  observation,  especially  in  the  making  of  anecdotes ; 
but  theoretically  the  two  problems  are  distinct.  And  at  the 
outset  of  our  discussion  of  the  former,  we  are  obliged  to 
acknowledge  that  all  psychic  interpretation  of  animal  behavior 
must  be  on  the  analogy  of  human  experience.  We  do  not 
know  the  meaning  of  such  terms  as  perception,  pleasure, 
fear,  anger,  visual  sensation,  etc.,  except  as  these  processes 
form  a  part  of  the  contents  of  our  own  minds.  Whether  we 
will  or  no,  we  must  be  anthropomorphic  in  the  notions  we 
form  of  what  takes  place  in  the  mind  of  an  animal.  Accept- 
ing this  fundamental  proposition,  the  students  of  animals 
have  yet  differed  widely  in  the  conclusions  they  have  drawn 
from  it.  Some  have  gone  to  the  extreme  of  declaring  that 
comparative  psychology  is  therefore  impossible.  Others 
have  joyfully  hastened  to  make  animals  as  human  as  they 
could.  Still  others  have  occupied  an  intermediate  position. 

Descartes  and  Montaigne  are  the  two  writers  antedating 
the  modern  period  who  are  most  frequently  quoted  in  this 
connection.  The  latter  had  evidently  a  natural  sympathy 
with  animals.  In  that  most  delightful  twelfth  chapter  of  the 


14  The  Animal  Mind 

second  book  of  Essays,  "  An  Apology  of  Raymond  Sebonde," 
he  gives  free  rein  to  the  inclination  to  humanize  them.  I 
quote  Florio's  translation:  "The  Swallowes  which  at  the 
approach  of  spring  time  we  see  to  pry,  to  search  and  ferret 
all  the  corners  of  our  houses;  is  it  without  judgment  they 
seeke,  or  without  discretion  they  chuse  from  out  a  thousand 
places,  that  which  is  fittest  for  them,  to  build  their  nests  and 
lodging?  .  .  .  Would  they  (suppose  you)  first  take  water 
and  then  clay,  unlesse  they  guessed  that  the  hardnesse  of  the 
one  is  softned  by  the  moistness  of  the  other?  .  .  .  Why 
doth  the  spider  spin  her  artificiall  web  thicke  in  one  place  and 
thin  in  another?  And  now  useth  one,  and  then  another 
knot,  except  she  had  an  imaginary  kind  of  deliberation,  fore- 
thought, and  conclusion?"  To  ascribe  such  behavior  to  the 
working  of  mere  instinct,  "with  a  kinde  of  unknowne, 
naturall  and  servile  inclination,"  is  unreasonable.  "The 
Fox,  which  the  inhabitants  of  Thrace  use  "  to  test  the  ice  on  a 
river  before  crossing,  which  listens  to  the  roaring  of  the  water 
underneath  and  so  judges  whether  the  ice  is  safe  or  not; 
"might  not  we  lawfully  judge  that  the  same  discourse  pos- 
sesseth  her  head  as  in  like  case  it  would  ours  ?  And  that  it  is 
a  kinde  of  debating  reason  and  consequence,  drawne  from 
natural  sense?  'Whatsoever  maketh  a  noyse  moveth, 
whatsoever  moveth,  is  not  frozen,  whatsoever  is  not  frozen, 
is  liquid ;  whatsoever  is  liquid,  yeelds  under  any  weight  ? ' ' 

(277). 

Descartes,  on  the  other  hand,  writing  some  sixty  years 
later,  takes,  as  is  well  known,  the  opposite  ground.  He 
says  in  a  letter  to  the  Marquis  of  Newcastle,  "  As  for  the  under- 
standing or  thought  attributed  by  Montaigne  and  others  to 
brutes,  I  cannot  hold  their  opinion."  While  animals  surpass 
us  in  certain  actions,  it  is,  he  holds,  only  in  those  "which 
are  not  directed  by  thought.  .  .  .  They  act  by  force  of 


Difficulties  and  Methods  15 

nature  and  by  springs,  like  a  clock,  which  tells  better  what  the 
hour  is  than  our  judgment  can  inform  us.  And  doubtless 
when  swallows  come  in  the  spring,  they  act  in  that  like  clocks. 
All  that  honey  bees  do  is  of  the  same  nature  "  (99,  pp.  281- 
283).  The  statement  of  Descartes,  contained  in  the  letter 
to  Mersenne  of  July  30,  1640,  that  animals  are  automata,  is 
often  misunderstood.  Descartes  does  not  assert  that  animals 
are  unconscious  in  the  sense  which  that  term  would  carry 
to-day,  but  only  that  they  are  without  thought.  Sensations, 
feelings,  passions,  he  is  willing  to  ascribe  to  them,  in  so  far  as 
these  do  not  involve  thought.  "  It  must  however  be  observed 
that  I  speak  of  thought,  not  of  life,  nor  of  sensation,"  he  says 
in  the  letter  to  Henry  More,  1649;  "I  do  not  refuse  to  them 
feeling  ...  in  so  far  as  it  depends  only  on  the  bodily 
organs  "  (99,  p.  287).  In  this  he  does  not  go  so  far  as  some 
modern  writers,  who  decline  to  assert  the  presence  of  any 
psychic  process  in  the  lower  forms  of  animal  life. 

Turning  to  recent  times,  we  find  arguments  very  like  those 
of  Montaigne  used  by  the  earlier  evolutionary  writers. 
Darwin,  for  instance,  says  in  "The  Descent  of  Man,"  "As 
dogs,  cats,  horses,  and  probably  all  the  higher  animals,  even 
birds,  have  vivid  dreams,  and  this  is  shown  by  their  move- 
ments and  the  sounds  uttered,  we  must  admit  that  they  pos- 
sess some  power  of  imagination"  (89,  p.  74).  "Even  brute 
beasts,"  says  Montaigne,  "...  are  seen  to  be  subject  to  the 
power  of  imagination;  witnesse  some  Dogs  .  .  .  whom  we 
ordinarily  see  to  startle  and  barke  in  their  sleep"  (277,  Bk.  I, 
ch.  20).  "  Only  a  few  persons,"  Darwin  continues,  "now  dis- 
pute that  animals  possess  some  power  of  reasoning.  Ani- 
mals may  constantly  be  seen  to  pause,  deliberate,  and  resolve." 
And  he  states  that  his  object  in  the  third  chapter  of  the  work 
quoted  is  "to  show  that  there  is  no  fundamental  difference 
between  man  and  the  higher  mammals  in  their  mental  facul- 


1 6  The  Animal  Mind 

ties"  (89,  p.  66).     Romanes  is  evidently  guided  by  the  same 
desire  to  humanize  animals. 

Now  these  writers  were  not  led  to  take  such  an  attitude 
merely  out  of  general  sympathy  with  the  brute  creation,  like 
Montaigne ;  they  had  an  ulterior  motive ;  namely,  to  meet  the 
objection  raised  in  their  time  against  the  doctrine  of  evolu- 
tion, based  on  the  supposed  fact  of  a  great  mental  and  moral 
gulf  between  man  and  the  lower  animals.  They  wished  to 
show,  as  Darwin  clearly  states,  that  this  gulf  is  not  absolute 
but  may  conceivably  have  been  bridged  by  intermediate  stages 
of  mental  and  moral  development.  While  this  argument 
against  evolution  was  being  pressed,  the  evolutionary  writers 
were  very  unsafe  guides  in  the  field  of  animal  psychology, 
for  they  distinctly  "held  a  brief  for  animal  intelligence," 
to  use  Thorndike's  phrase.  In  more  recent  times  interest  in 
both  the  positive  and  the  negative  sides  of  the  objection  drawn 
from  man's  superiority  has  died  out,  and  such  special  plead- 
ing has  become  unnecessary. 

•\  On  the  other  hand,  the  fact  that  the  greater  part  of  the 
experiments  on  animals  were  until  the  last  ten  or  fifteen  years 
performed  by  physiologists  has  given  rise  to  an  opposite  ten- 
dency in  interpreting  the  animal  mind:  the  tendency  to 
make  purely  biological  concepts  suffice  as  far  as  possible 
for  the  explanation  of  animal  behavior  and  to  assume  the 
presence  even  of  consciousness  in  animals  only  when  it  is 
absolutely  necessary  to  do  so.  °Loeb  in  1890  suggested  the 
theory  which  he  has  since  elaborated,  that  the  responses 
of  animals  to  stimulation,  instead  of  being  signs  of  "  sensa- 
tion," are  in  every  way  analogous  to  the  reactions  of  plants 
to  such  forces  as  light  and  gravity;  hence  unconscious  "trop- 
isms"  (235). "  Bethe  in  1898  attempted  to  explain  all  the  com- 
plicated behavior  of  ants  and  bees,  which  the  humanizing 
writers  had  compared  with  our  own  civilization,  as  a  result  of 


Difficulties  and  Methods  17 

reflex  responses,  chiefly  to  chemical  stimulation,  unaccompa- 
nied by  any  consciousness  whatever  (30).  This  revival,  in  an 
altered  form,  of  the  Cartesian  doctrine  has  met  with  energetic 
opposition,  especially  from  writers  having  philosophical 
interests.  At  the  present  time  the  parties  in  the  controversy 
may  be  divided  into  three  groups:  those  who  believe  that 
consciousness  should  be  ascribed  to  all  animals;  those  who 
hold  that  it  should  be  ascribed  only  to  those  animals  whose 
behavior  presents  certain  peculiarities  regarded  as  evidence  of 
mind;  and  those  who  hold  that  we  have  no  trustworthy 
evidence  of  mind  in  any  animal,  and  should  therefore  abandon 
comparative  psychology  and  use  only  physiological  terms. 

To  the  first  group  belong,  among  others,  the  French  writer 
Claparede,  the  Swiss  naturalist  Forel,  and  the  Jesuit  Was- 
mann.  The  physiologist  W.  A.  Nagel  also  takes  a  friendly 
attitude  toward  the  animal  mind.  In  the  second  group  may 
be  classed  Loeb  and  H.  Jordan.  In  the  third  belong  the 
physiologists  Beer,  Bethe,  H.  E.  Ziegler,  von  Uexkiill,  and 
J.  P.  Nuel. 

Claparede,  Forel,  and  Wasmann  maintain  the  existence  of 
consciousness  in  animals  from  widely  different  philosophical 
points  of  view.  The  first-named  is  what  is  called  a  parallel- 
ist;  that  is,  he  believes  that  mental  processes  and  bodily 
processes  are  not  causally  related,  but  form  two  parallel 
and  non-interfering  series  of  events.  In  the  study  of  animals, 
both  the  physical  and  the  psychical  series  should,  he  thinks, 
be  investigated.  Biology  should  use  two  parallel  methods: 
the  one  ascending,  attempting  to  explain  animal  behavior  by 
physical  and  chemical  laws;  the  other  descending,  giving 
an  account  of  the  mental  processes  of  animals.  Ultimately, 
it  may  be  hoped,  according  to  Claparede,  that  both  methods 
will  be  applied  throughout  the  whole  range  of  animal  life. 
At  present  the  ascending  method  is  most  successful  with  the 


1 8  The  Animal  Mind 

lowest  forms,  the  descending  method  with  the  highest  forms. 
We  cannot  afford  to  abandon  the  psychological  study  of 
animals,  for  our  knowledge  of  the  nervous  processes  under- 
lying the  higher  mental  activities  is  very  slight;  physiology 
here  fails  us,  and  psychology  must  be  left  in  command  of 
the  field.  The  danger  besetting  the  attempt  at  a  purely 
physical  explanation  of  animal  behavior  is  that  the  facts  shall 
be  unduly  simplified  to  fit  the  theory.  Thus  Bethe's  effort 
at  explaining  the  way  in  which  bees  find  their  way  back  to 
the  hive  as  a  reflex  response,  or  tropism,  produced  by  "an 
unknown  force,"  is  highly  questionable;  the  facts  seem  to 
point  toward  the  exercise  of  some  sort  of  memory  by  the  bees. 
It  is  always  possible,  further,  that  the  tropism  is  accompanied 
by  consciousness.  A  physiologist  from  Saturn  might  reduce 
all  human  activities  to  tropisms,  says  Claparede  in  a  striking 
passage.  "The  youth  who  feels  himself  drawn  to  medical 
studies,  or  he  who  is  attracted  to  botany,  can  no  more  account 
for  his  profoundest  aspirations  than  the  beetle  which  runs 
to  the  odor  of  a  dead  animal  or  the  butterfly  invited  by  the 
flowers ;  and  if  the  first  shows  a  certain  feeling  corresponding 
to  these  secret  states  of  the  organifm  (a  feeling  of  '  predilec- 
tion' for  such  a  career,  etc.),  how  can  we  dare  to  deny  to  the 
second  analogous  states  of  consciousness?"  (75).  If  it  is 
argued  that  we  have  no  direct,  but  only  an  inferential,  knowl- 
edge of  the  processes  in  an  animal's  mind,  the  argument  is 
equally  valid  against  human  psychology,  for  'the  psychologist 
has  only  an  inferential  knowledge  of  his  neighbor's  mind  (77). 
Wasmann  defends  the  animal  mind  from  a  different  point  of 
view.  For  one  thing,  he  believes  that  mental  processes  may 
act  causally  upon  bodily  states.  He  accepts,  in  other  words, 
what  is  called  iriteractionism,  as  opposed  to  parallelism. 
Further,  although  he  strongly  opposes  the  doctrineTnatthe 
reactions  of  animals  are  unconscious  tropisms,  and  constantly 


Difficulties  and  Methods  19 

emphasizes  their  variability  and  modifiability  through  experi- 
ence, he  nevertheless  believes  that  a  gulf  separates  the  human 
from  the  animal  mind.  The  term  ''intelligence"  which 
most  writers  use  to  designate  merely  the  power  of  learning 
by  individual  experience,  Wasmann  would  reserve  for  the 
power  of  deducing  and  understanding  relations,  and  would 
assign  only  to  human  beings  (425,  426).  Although  animals 
have  their  instincts  modified  by  sense  experience,  man 
"stands  through  his  reason  and  freedom  immeasurably  high 
above  the  irrational  animal  that  follows,  and  must  follow, 
its  sensuous  impulse  without  deliberation  "  (425). 

Forel,  in  the  third  place,  is  what  is  .called  a  monist  in  meta- 
physics. That  is,  he  does  not  believe  either  that  mind  and 
body  are  parallel,  or  that  they  interact  causally,  but  that  they 
are  two  aspects  of  the  same  reality.  "Every  psychic  phe- 
nomenon is  the  same  real  thing  as  the  molecular  or  neurocymic 
activity  of  the  brain-cortex  coinciding  with  it "  (132,  p.  7). 
The  psychic  and  the  physical,  on  this  theory,  should  be 
coextensive;  not  merely  should  consciousness  in  some  form 
belong  to  all  living  things,  but  every  atom  of  matter  should 
have  its  psychic  aspect,  •n  such  a  basis,  Forel  takes  highly 
optimistic  views  of  the  animal  mind.  In  insects,  of  which  he 
has  made  a  special  study,  it  is,  he  thinks,  "possible  to  demon- 
strate the  existence  of  memory,  associations  of  sensory  images, 
perceptions,  attention,  habits,  simple  powers  of  inference 
from  analogy,  the  utilization  of  individual  experience,  and 
hence  distinct,  though  feeble,  plastic  individual  deliberations 
or  adaptations  "  (132,  p.  36). 

The  second  of  the  three  groups  into  which  we  divided 
present-day  writers  on  the  interpretation  of  animal  behavior 
contains  those  who  maintain  not  that  all  animals  are  conscious, 
but  that  those  whose  behavior  meets  a  certain  standard  may 
be  so  considered.  The  nature  of  this  test  is  a  difficult  prob- 


2O  The  Animal  Mind 

lem.  We  shall  therefore  devote  the  next  chapter  to  its  con- 
sideration; and  as  it  necessarily  plays  an  important  part  in 
determining  views  regarding  the  animal  mind,  we  shall ' 
postpone  for  the  present  the  discussion  of  the  second  group. 
The  third  group  contains  those  biologists,  conservative  or 
radical  according  to  one's  own  position,  who  deny  to  cojipar- 
ative_^s^chologyjhejightjo  exist.  The  eminent  neurologist 
Bethe  is  a  typical  representative  of  the  class.  In  his  study  of 
the  behavior  of  ants  and  bees  he  refuses  to  allow  these  animals 
any  "psychic  qualities"  whatever,  and  suggests  the  term 
"chemo-reception"  instead  of  "smell,"  to  designate  the  in- 
fluence which  directs  most  of  their  reactions,  —  " smell"  im- 
plying a  psychic  quality  (30).  From  his  argument  for  the 
probable  absence  of  consciousness  in  ants  and  bees,  as  well 
as  in  the  crab  (28),  one  might  be  inclined  to  put  Bethe  in  the 
second  of  the  above-mentioned  classes,  for  it  is  the  lack  of  one 
definite  characteristic  in  the  behavior  of  these  animals,  namely, 
modification  by  individual  experience,  that  makes  him  think 
them  unconscious.  It  becomes  clear  from  other  passages  in 
his  writings,  however,  that  he  considers  the  presence  of  con- 
sciousness even  in  animals  that  fein  learn  by  experience,  a 
highly  problematical  and  improper  assumptidr!.  In  a  foot- 
note to  a  later  article  he  says:  " Psychic  qualities  cannot  be 
demonstrated.  Even  what  we  call  sensation  is  known  to 
each  man  only  in  himself,  since  it  is  something  subjective. 
We  possess  the  capacity  of  modifying  our  behavior  [i.e.  of 
learning],  and  every  one  knows  from  his  own  experience  that 
psychic  qualities  play  a  part  connected  with  this  modifying 
process.  Every  statement  that  another  being  possesses 
psychic  qualities  is  a  conclusion  from  analogy,  not  a  cer- 
tainty; it  is  a  matter  of  faith.  If  one  wishes  to  draw  this 
analogical  inference,  it  should  be  made  where  the  capacity 
for  modification  can  be  shown.  When  this  is  lacking,  there 


Difficulties  and  Methods  21 

is  not  the  slightest  scientific  justification  for  assuming  psychic 
qualities.  They  may  exist,  but  there  is  no  probability  of  it, 
and  hence  science  should  deny  them.  Hence  if  one  ventures 
to  speak  of  a  Psyche  in  animals  at  all,  one  should  give  the 
preference  to  those  which  can  modify  their  behavior  "  (29). 
But  that  Bethe  himself  prefers  not  to  make  the  venture  is 
evident  from  statements  in  .the  text  of  the  same  article.  The 
psychic  or  subjective,  he  says,  is  unknowable,  and  the  only 
thing  we  may  hope  to  know  anything  about  is  the  chemical 
and  physiological  processes  involved.  "These  chemo-physi- 
cal  processes  and  their  consequences,  that  is,  the  objective 
aspect  of  psychic  phenomena,  and  these  alone,  should  be  the 
object  of  scientific  investigation  "  (29). 

Together  with  Beer  and  von  Uexkiill,  Bethe  shortly  after- 
ward published  "  Proposals  for  an  Objectifying  Nomen- 
clature in  the  Physiology  of  the  Nervous  System."  The 
main  purpose  of  this  paper  was  to  suggest  that  all  terms 
having  a  psychological  implication,  such  as  sight,  smell, 
sense-organ,  memory,  learning,  and  the  like,  be  carefully 
excluded  from  discussions  of  animal  reactions  to  stimulation 
and  animal  behavior  generally.  In  their  stead  the  authors 
propose  such  expressions  as  the  following:  for  responses  to 
stimulation  where  no  nervous  system  exists,  the  term  anti- 
types; for  those  involving  a  nervous  system,  antikineses ; 
the  latter  are  divided  into  reflexes,  where  the  response  is  uni- 
form, and  antiklises,  where  the  response  is  modifiable.  A 
sense-organ  becomes  a  reception-organ,  sensory  nerves  are 
receptory-nerves,  and  we  have  phono-reception,  stibo-reception, 
photo-reception,  instead  of  hearing,  smell,  and  sight.  The 
after-effect  of  a  stimulus  upon  later  ones  is  the  resonance 
of  the  stimulus  .(20). 

This  attempt  at  an  objective  terminology  meets  the  cor- 
dial approval  of  H.  E.  Ziegler  and  J.  P.  Nuel.  The  former 


22  The  Animal  Mind 

declares  that  the  concept  of  consciousness  is  worthless  in  the 
study  of  animals,  as  no  one  knows  whether  an  animal  is 
conscious  or  not.  He  suggests  as  additions  to  the  new  vocab- 
ulary the  term  pleronomic  to  designate  inherited  reactions,  and 
enbiontic  to  signify  acquired  reactions  (476).  Nuel  also 
thinks  that  our  ignorance  of  the  mental  states  of  animals 
renders  comparative  psychology  unscientific.  He  prefers  the 
"kinetic"  to  the  psychic  point  of  view;  a  sense-organ,  in 
man  or  beast,  is  an  apparatus  for  reactions  (297).  In  a  book 
on  vision  Nuel  has  suggested  an  objective  terminology  of  his 
own,  where  "ikonoreaction,"  for  example,  takes  the  place  of 
sight  (2  96).* 

It  would  seem  that  no  serious  objection  could  be  raised 
against  the  use  of  a  purely  objective  nomenclature  in  physiol- 
ogy, and  that  confusion  might  thereby  be  avoided,  without 
prejudicing  the  case  of  comparative  psychology,  which  might 
exist  side  by  side  with  the  other  science,  and  reserve  the  terms 
with  psychic  implications  for  itself.  Wasmann,  it  is  true, 
objects  to  the  new  terminology  on  its  own  account,  as  cum- 
brous and  scholastic,  and  says  that  if  Ziegler  cannot  use  such 
wprds  as  sensation,  perception,  seeing,  and  the  like,  without 

*  anthropomorphism,  the  fault  is  his  own,  and  the  fact  should 
not  lead  him  to  impose  a  new  set  of  words  on  others  (428). 
These  criticisms,  however,  are  those  of  the  conservative  who 
objects  to  anything  new;  all  technical  vocabularies  are  pe- 
dantic, but  it  is  impossible  to  take  too  many  precautions 
against  confusion  of  ideas. 

^^What  attitude,  now,  shall  we  assume  upon  the  broader 
question  raised  by  these  writers,  as  to  whether  comparative 
psychology  is  possible  at  all  ?  Must  we  accept  the  statement 

1  Another  instance  of  an  attempt  to  use  terminology  without  psychic  im- 
plications is  to  be  found  in  R.  Semon's  "Die  Mncme  als  erhaltendes 
Princip  im  Wechsel  des  organischen  Geschehens  "  (379). 


Difficulties  and  Methods  23 

that  no  knowledge  whatever  of  the  animal  mind  is  obtain- 
able? If  so,^we  must  also  admit  that  human  psychology  is 
impossible.  Our  acquaintance  with  the 'mind  of  animals 
rests  upon  the  same  basis  as  our  acquaintance  with  the  mind 
of  our  fellow-man;  both  are  derived  by  inference  from 
observed  behavior.  The  actions  of  our  fellow-men  resemble 
our  own,  and  we  therefore  infer  in  them  like  subjective  states 
to  ours :  the  actions  of  animals  resemble  ours  less  completely, 
"but  the  difference  is  one  of  degree,  not  of  kind.  This  argu- 
ment in  behalf  of  comparative  psychology,  which  is  brought 
forward  by  Claparede  (77),  is  opposed  by  Nuel  with  the 
denial  that  human  and  animal  psychology  rest  upon  the  same 
basis  (297);  but  no  cogent  proofs  accompany  Nuel's  state- 
ment. The  physiologists  would  doubtless  accept  the  other 
horn  of  the  dilemma,  and  reject  human  psychology  along  with 
animal  psychology ;  but  a  scientific  rigor  which  requires  of  us 
to  abandon  the  assertion  of  mind  in  our  fellow-beings  and 
the  study  of  that  mind  has  pushed  itself  to  absurdity.  As 
Jordan  says,  inferences  upon  a  basis  of  probability  form  a 
legitimate  part  of  science  (216).  The  mental  processes  in 
other  minds,  animal  or  human,  cannot  indeed  be  objectively 
ascertained  facts ;  the  facts  are  those  of  human  and  animal 
behavior;  but  the  mental  processes  are  as  justifiable  infer- 
ences as  any  others  with  which  science  deals.  The  prime 
necessity  is  merely  that  they  shall  be  properly  guarded.  Cer-X 
tain  precautions  are  necessary  when  we  infer  the  state  of  our  j 
neighbor's  mind;  certain  added  precautions  are  necessary  / 
when  we  infer  states  in  the  mind  of  an  animal,  and  our  asy 
sertions  should  certainly  diminish  in  dogmatism  as  we  go 
down  the  scale  of  animal  life.  But  the  psychologist,  to  whom, 
as  Titchener  has  put  it,  uthe  facts  and  laws  of  mind  are  the 
most  real  things  that  the  world  can  show  "  (400),  will  never 
consent  to  abandon  the  effort  to  probe  the  mysteries  of  other 


24  The  Animal  Mind 

minds,  human  or  animal,  until  science  consents  to  abandon 
all  hypotheses  and  inferences  based  on  anything  short  of 
perfect  identity  between  instances. 

Is  it  possible  to  state  briefly  the  special  precautions  that 
must  be  observed  in  interpreting  animal  behavior  as  accom- 
panied by  consciousness,  granted  that  such  interpretation  is 
admissible  ?  Jordan,  while  holding  that  the  existence  of  the 
animal  mind  may  fairly  be  inferred  under  certain  circum- 
stances, holds  that  we  are  not  justified  in  inferring  the  actual 
quality  of  mental  processes  in  animals.  For  this  reason  he 
objects  to  the  term  "  comparative  psychology"  (216).  There 
is  no  doubt  that  great  caution  should  be  used  in  regarding  the 
quality  of  a  human  conscious  process  as  identical  with  the 
quality  of  the  corresponding  process  in  the  animal  mind.  For 
example,  we  might  say  with  a  fair  degree  of  assurance  that 
an  animal  consciously  discriminates  between  light  and 
darkness ;  that  is,  receives  conscious  impressions  of  different 
quality  from  the  two,  yet  the  mental  impression  produced  by 
white  light  upon  the  animal  may  be  very  different  frornjfee 
sensation  of  white  as  we  know  it,  and  the  impression  procmced 
by  the  absence  of  light  very  different  from  our  sensation  of 
black.  Black  and  white  may,  for  all  we  know,  depend  for 
their  quality  upon  some  substance  existing  only  in  the  human 
retina. 

A  second  precaution  concerns  the  simplicity  or  complexity 
of  the  interpretation  put  upon  animal  behavior.  Lloyd 
Morgan,  in  his  "  Introduction  to  Comparative  Psychology," 
formulated  a  conservative  principle  of  interpretation  which 
has  often  been  quoted  as  " Lloyd  Morgan's  Canon."  The 
principle  is  as  follows :  "  In  no  case  may  we  interpret  an  action 
as  the  outcome  of  the  exercise  of  a  higher  psychical  faculty, 
if  it  can  be  interpreted  as  the  outcome  of  the  exercise  of  one 
which  stands  lower  hi  the  psychological  scale"  (280,  p.  53). 


Difficulties  and  Methods  25 

In  other  words,  when  in  doubt  take  the  simpler  interpretation. 
For  example,  a  dog  detected  in  a  theft  cowers  and  whines.  One 
possible  mental  accompaniment  of  this  behavior  is  remorse; 
the  dog  is  conscious  that  he  has  fallen  below  a  moral  standard, 
and  grieved  or  offended  his  master. .  A  second  is  the  antici- 
pation of  punishment ;  the  dog  has  a  mental  representation  of 
the  consequences  of  his  action  upon  former  occasions,  and 
imagining  himself  likely  to  experience  them  anew,  is  terrified 
at  the  prospect.  $  A  third  possibility  is  that  the  dog's  previous 
experience  of  punishment,  instead  of  being  revived  in  the  form 
of  definite  images,  makes  itself  effective  merely  in  his  feelings 
and  behavior;  he  is  uncomfortable  and  frightened,  he  knows 
not  definitely  why.  It  is  evident  that  these  three  possibili- 
ties represent  three  different  grades  of  complexity  of  mental 
process,  the  first  being  by  far  the  highest.  Lloyd  Morgan's 
canon  enjoins  upon  us  irj  such  a  case  to  prefer  the  third 
alternative,  provided  that  it  will  really  account  for  the  dog's 
behavior. 

Now  why  should  the  simplest  interpretation  be  pre- 
ferred? We  must  not  forget  that  the  more  complex  ones 
remain  in  the  field  of  possibility.  Positive  assertions  have  no 
place  in  comparative  psychology.  We  cannot  say  that  the 
simplicity  of  an  hypothesis  is  sufficient  warrant  of  its  truth, 
for  nature  does  not  always  proceed  by  the  paths  which  seem 
to  us  least  complicated.  The  fact  is  that  Lloyd  Morgan's 
principle  serves  to  counterbalance  our  most  important  source 
of  error  in  interpreting  animal  behavior.  It  is  like  tipping 
a  boat  in  one  direction  to  compensate  .for  the  fact  that  some 
one  is  pulling  the  opposite  gunwale.  /We  must  interpret  the 
animal  mind  humanly  if  we  are  to  interpret  it  at  all.  Yet  we 
know  that  it  differs  from  the  human  mind,  and  that  the  dif- 
ference  is  partly  a  matter  of  complexity.  Let  us  therefore 
take  the  least  complex  interpretation  that  the  facts  of  animal 


26  The  Animal  Mind 

behavior  will  admit,  always  remembering  that  we  may  be 
wrong  in  so  doing,  but  resting  assured  that  we  are,  upon  the 
whole,  on  the  safer  side.  The  social  consciousness  of  man  is 
very  strong,  and  his  tendency  to  think  of  other  creatures, 
even  of  inanimate  nature,  as  sharing  his  own  thoughts  and 
feelings,  has  shown  itself  in  his  past  to  be  almost  irresistible. 
Lloyd  Morgan's  canon  offers  the  best  safeguard  against  this 
natural  inclination,  short  of  abandoning  all  attempt  to  study 
the  mental  life  of  the  lower  animals. 


CHAPTER  II 
THE  EVIDENCE  OF  MIND 

§  6.    Inferring  Mind  from  Behavior 

IN  the  last  chapter  we  saw  that  some  recent  writers  uj 
animal  behavior  and  its  interpretation,  while  refusing  to  ad- 
mit the  presence  of  consciousness  in  all  forms  of  animal  life, 
yet  hold  that  it  can  be  proved  to  exist  in  certain  forms.  The 
latter,  it  is  maintained,  display  certain  peculiarities  of  be- 
havior that  may  be  regarded  as  proofs  of  a  psychic  accompani- 
ment. Into  the  nature  of  these  proofs  we  may  now  inquire. 

To  begin  with,  can  it  be  said  that  when  an  animal  makes  a 
movement  in  response  to  a  certain  stimulus,  there  is  an  accom- 
panying consciousness  of  the  stimulus,  and  that  when  it  fails  ^ 
to  move,  there  is  no  consciousness?  Is  response  to  stimu- 
lation evidence  of  consciousness  ?  In  the  case  of  man,  we  / 
know  that  absence  of  visible  response  does  not  prove  that  the 
stimulus  has  not  been  sensed ;  while  it  is  probable  that  some 
effect  upon  motor  channels  always  occurs  when  consciousness 
.accompanies  stimulation,  the  effect  may  not  be  apparent  to 
an  outside  observer.  On  the  other  hand,  if  movement  in 
response  to  the  impact  of  a  physical  force  is  evidence  of  con- 
sciousness, then  the  ball  which  falls  under  the  influence  of 
gravity  and  rebounds  on  striking  the  floor  is  conscious.  Nor 
is  the  case  improved  if  we  point  out  that  the  movements  which 
animals  make  in  response  to  stimulation  are  not  the  equiva- 
lent in  energy  of  the  stimulus  applied,  but  involve  the  setting 
free  of  energy  stored  in  the  animal  as  well.  True,  when  a 
microscopic  animal  meets  an  obstacle  in  its  swimming,  and 

27 


28  The  Animal  Mind 

darts  backward,  the  movement  is  not  a  mere  rebound;  it 
implies  energy  contributed  by  the  animal's  own  body.  But 
just  so  an  explosion  of  gunpowder  is  not  the  equivalent  in 
energy  of  the  heat  of  the  match,  the  stimulus.  Similarly  it 
is  possible  to  think  of  the  response  made  by  animals  to  external 
stimuli  as  involving  nothing  more  than  certain  physical  and 
chemical  processes  identical  with  those  existing  in  inanimate 
nature. 

If  we  find  that  the  movements  made  by  an  animal  as  a  re- 
sult of  external  stimulation  regularly  involve  withdrawal  from 
certain  stimuli  and  acceptance  of  others,  it  is  natural  to  use 
the  term  "  choice  "  in  describing  such  behavior.  But|if  con- 
sciousness is  supposed  to  accompany  the  exercise  of  choice  in 
*J  this  sense,  then  consciousness  must  be  assumed  to  accompany 
/, '•''the  behavior  of  atoms  in  chemical  combinations.)  When 
hydrochloric  acid  is  added  to  a  solution  of  silver  nitrate,  the 
atoms  of  chlorine  and  those  of  silver  find  each  other  by  an 
unerring  " instinct"  and  combine  into  the  white  precipitate  of 
silver  chloride,  while  the  hydrogen  and  the  nitric  acid  simi- 
larly "choose"  each  other.  Nor  can  the  fact  that  behavior 
in  animals  is  adapted  to  an  end  be  used  as  evidence  of  mind ; 
for  " purposive"  reactions,  which  contribute  to  the  welfare 
of  an  organism,  are  themselves  selective.  The  search  for 
food,  the  care  for  the  young,  and  the  complex  activities  which 
further  welfare,  are  made  up  of  reactions  involving  "choice" 
between  stimuli;  and  if  the  simple  "choice"  reaction  is  on  a 
par  with  the  behavior  of  chemical  atoms,  so  far  as  proof  of 
consciousness  goes,  then  adaptation  to  an  end,  apparent  pur- 
posiveness,  is  in  a  similar  position. 

Thus  the  mere  fact  that  an  animal  reacts  to  stimulation, 
even  selectively  and  for  its  own  best  interests,  offers  no  evi- 
dence for  the  existence  of  mind  that  does  not  apply  equally 
well  to  particles  of  inanimate  matter.  Moreover,  there  is 


The  Evidence  of  Mind  29 

some  ground  for  holding  that  the  reactions  of  the  lowest 
animals  are  unconscious.  This  ground  consists  in  the  ap- 
parent lack  of  variability  which  characterizes  such  reactions. 
In  our  own  case,  we  know  that  certain  bodily  movements, 
those  of  digestion  and  circulation,  for  example,  are  normally 
carried  on  without  accompanying  consciousness,  and  that  in 
other  cases  where  there  is  consciousness  of  the  stimulus,  as 
in  the  reflex  knee-jerk,  it  occurs  after  the  movement  is  initiated, 
so  that  the  nervous  process  underlying  the  sensation  would 
seem  to  be  immaterial  to  the  performance  of  the  movement. 
These  unconscious  reactions  in  human  beings  are  character- 
ized by  their  relative  uniformity,  by  the  absence  of  variation 
in  their  performance.  Moreover,  when  an  action  originally 
accompanied  by  consciousness  is  often  repeated,  it  tends,  by 
what  is  apparently  one  and  the  same  process,  to  become  un- 
conscious and  to  .become  uniform.  There  is  consequently 
reason  for  believing  that  when  the  behavior  of  lower  animals 
displays  perfect  uniformity,  consciousness  is  not  present.  On 
the  other  hand,  an  important  reservation  must  be  made  in  the 
use  of  this  negative  test.  It  is  by  no  means  easy  to  be  sure 
that  an  animal's  reactions  are  uniform.  The  more  carefully 
the  complexer  ones  are  studied,  the  more  are  variability  and 
difference  brought  to  light  where  superficial  observation  had 
revealed  a  mechanical  and  automatic  regularity.  It  is  quite  ^ 
possible  that  even  in  the  simple,  apparently  fixed  response  of 
microscopic  animals  to  stimulation,  better  facilities  for  ob- 
servation might  show  variations  that  do  not  now  appear. 

This  matter  of  uniformity  versus  variability  suggests  a 
further  step  in  our  search  for  a  satisfactory  test  of  the  presence 
of  mind.  Is  mere  variability  in  behavior,  mere  irregularity 
in  response,  to  be  taken  as  such  a  test  ?  Not  if  we  argue  from 
our  own  experience.  While  that  portion  of  our  own  be- 
havior which  involves  consciousness  shows  more  irregularity 


30  The  Animal  Mind 

than  the  portion  which  does  not,  yet  the  causes  of  the  irregu- 
larity are  often  clearly  to  be  found  in  physiological  conditions 
with  which  consciousness  has  nothing  to  do.  There  are  days 
when  we  can  think  clearly  and  recall  easily,  and  days  when 
obscurities  refuse  to  vanish  and  the  right  word  refuses  to  come ; 
days  when  we  are  irritable  and  days  when  we  are  sluggish. 
Yet  since  we  can  find  nothing  in  our  mental  processes  to  ac- 
count for  this  variability,  it  would  be  absurd  to  take  analogous 
fluctuations  in  animal  behavior  as  evidence  of  mind.  So 
complicated  a  machine  as  an  animal  organism,  even  if  it  be 
nothing  more  than  a  machine,  must  show  irregularities  in  its 
working. 

Behavior,  then,  must  be  variable,  but  not  merely  variable, 
to  give  evidence  of  mind.  The  criterion  most  frequently  ap- 
plied to  determine  the  presence  or  absence  of  the  psychic  is 
a  'variation  in  behavior  that  shows  definitely  the  result  of 
previous  individual  experience.  "Does  the  organism,"  says 
Romanes,  "learn  to  make  new  adjustments,  or  to  m6dify  old 
ones,  in  accordance  with  the  results  of  its  own  individual  ex- 
perience ? "  (364,  p.  4).  Loeb  declares  that  "the  fundamental 
process  which  occurs  in  all  psychic  phenomena  as  the  ele- 
mental component "  is  "the  activity  of  the  associative  memory, 
or  of  association,"  and  defines  associative  memory  as  "that 
mechanism  by  which  a  stimulus  brings  about  not  only  the 
effects  which  its  nature  and  the  specific  structure  of  the  irritable 
organ  call  for,  but  by  which  it  brings  about  also  the  effects  of 
other  stimuli  which  formerly  acted  upon  the  organism  almost 
or  quite  simultaneously  with  the  stimulus  in  question." 
"If  an  animal  can  be  trained,"  he  continues,  "if  it  can  learn, 
it  possesses  associative  memory,"  and  therefore  mind  (243, 
p.  1 2).  The  psychologist  finds  the  term  "  associative  memory  " 
hardly  satisfactory,  and  objects  to  the  confusion  between 
mental  and  physical  concepts  which  renders  it  possible  to 


The  Evidence  of  Mind  31 

speak  of  a  " mechanism"  as  forming  an  "elemental  com- 
ponent" in  " psychic  phenomena,"  but  these  points  may  be 
passed  over.  The  power  to  learn  by  individual  experience 
is  the  evidence  which  Romanes,  Morgan,  and  Loeb  will 
accept  as  demonstrating  the  presence  of  mind  in  an  animal. 

Does  the  absence  of  proof  that  an  animal  learns  by  expe- 
rience show  that  the  animal  is  unconscious?  Romanes  is 
careful  to  answer  this  question  in  the  negative.  "Because  a 
lowly  organized  animal,"  he  says,  "does  not  learn  by  its  own 
individual  experience,  we  may  not  therefore  conclude  that  in 
performing  its  natural  or  ancestral  adaptations  to  appropriate 
stimuli,  consciousness,  or  the  mind  element,  is  wholly  absent ; 
we  can  only  say  that  this  element,  if  present,  reveals  no  evi- 
dence of  the  fact  "  (364,  p.  3).  Loeb,  on  the  other  hand,  writes 
as  if  absence  of  proof  for  consciousness  amounted  to  disproof, 
evidently  relying  on  the  principle  of  parsimony,  that  no 
unnecessary  assumptions  should  be  admitted.  "Our  crite- 
rion," he  remarks,  "puts  an  end  to  the  metaphysical  ideas 
that  all  matter,  and  hence  the  whole  animal  world,  possesses 
consciousness"  (243,  p.  13).  If  learning  by  experience  be 
really  a  satisfactory  proof  of  mind,  then  its  absence  in  certain 
animals  would  indeed  prevent  the  positive  assertion  that  all 
animals  are  conscious ;  but  it  could  not  abolish  the  possibility 
that  they  might  be.  Such  a  possibility  might,  however,  be 
of  no  more  scientific  interest  than  any  one  of  a  million  wild 
possibilities  that  science  cannot  spare  time  to  disprove.  But 
we  shall  find  that  learning  by  experience,  taken  by  itself,  is 
too  indefinite  a  concept  to  be  of  much  service,  and  that  when 
defined,  it  is  inadequate  to  bear  the  whole  weight  of  proving 
consciousness  in  animals.  Such  being  the  case,  the  possi- 
bility that  animals  which  have  not  been  shown  to  learn  may 
yet  be  conscious  acquires  the  right  to  be  reckoned  with. 

The  first  point  that  strikes  us  in  examining  the  proposed 


32  The  Animal  Mind 

test  is  that  the  learning  by  experience  must  not  be  too  slow, 
or  we  can  find  parallels  for  it  in  the  inanimate  world.  An 
animal  may  be  said  to  have  learned  by  experience  if  it  be- 
haves differently  to  a  stimulus  because  of  preceding  stimuli. 
But  it  is  one  thing  to  have  behavior  altered  by  a  single  pre- 
ceding stimulus,  and  another  to  have  it  altered  by  two  hun- 
dred repetitions  of  a  stimulus.  The  wood  of  a  violin  reacts 
differently  to  the  vibrations  of  the  strings  after  it  has  "expe- 
rienced" them  for  ten  years;  the  molecules  of  the  wood  have 
gradually  taken  on  an  altered  arrangement.  A  steel  rail  re- 
acts differently  to  the  pounding  of  wheels  after  that  process 
has  been  long  continued ;  it  may  snap  under  the  strain.  Shall 
we  say  that  the  violin  and  the  rail  have  learned  by  individual 
experience  ?  If  the  obvious  retort  be  made  that  it  is  only  in 
living  creatures  that  learning  by  experience  should  be  taken 
as  evidence  of  mind,  let  us  take  an  example  from  living  crea- 
tures. When  a  blacksmith  has  been  practising  his  trade  for 
a  year,  the  reactions  of  his  muscles  are  different  from  what 
they  were  at  the  outset.  But  this  difference  is  not  merely  a 
matter  of  more  accurate  sense-discrimination,  a  better  "  plac- 
ing" of  attention  and  the  like ;  there  have  been  going  on  within 
the  structure  of  his  muscles  changes  which  have  increased  their 
efficiency,  and  with  which  consciousness  has  had  nothing  to 
do.  These  changes  have  been  extremely  slow  compared  to 
the  learning  which  does  involve  consciousness.  In  one  or 
two  lessons  the  apprentice  learned  what  he  was  to  do;  but 
only  very  gradually  have  his  muscles  acquired  the  strength  to 
do  it  as  it  should  be  done.  Now  among  the  lower  animal 
forms  we  sometimes  meet  with  learning  by  experience  that  is 
very  slow ;  that  requires  a  hundred  or  more  repetitions  of  the 
stimulus  before  the  new  reaction  is  acquired.  In  such  a  case 
we  can  find  analogical  reasons  for  suspecting  that  a  gradual 
change  in  the  tissues  of  the  body  has  taken  place,  of  the  sort 


The  Evidence  of  Mind  33 

which,  like  the  attuning  of  the  violin  wood  or  the  slow  de- 
velopment of  a  muscle,  have  no  conscious  accompaniment. 
We  must  then  ask  the  question :  What  kind  of  learning 
by  experience  never,  so  far  as  we  know,  occurs  unconsciously  ? 
Suppose  a  human  being  shut  up  in  a  room  from  which  he  can 
escape  only  by  working  a  combination  lock.  As  we  shall  see 
later,  this  is  one  of  the  methods  by  which  the  learning  power 
of  animals  has  been  tested.  The  man,  after  prolonged 
investigation,  hits  upon  the  right  combination  and  gets  out. 
Suppose  that  he  later  finds  himself  again  in  the  same  pre- 
dicament, and  that  without  hesitation  or  fumbling  he  opens 
the  lock  at  once,  and  performs  the  feat  again  and  again,  to 
show  that  it  was  not  a  lucky  accident.  But  one  interpretation 
of  such  behavior  is  possible.  We  know  from  our  own  ex- 
perience that  the  man  could  not  have  worked  the  lock  the 
second  time  he  saw  it,  unless  he  consciously  remembered  the 
movements  he  made  the  first  time ;  that  is,  unless  he  had  in 
mind  some  kind  of  idea  as  a  guide.  Here,  at  least,  there  can 
have  been  no  change  in  the  structure  of  the  muscles,  for  such 
changes  are  gradual;  the  change  must  have  taken  place  in 
the  most  easily  alterable  portion  of  the  organism,  the  nervous 
system;  and  further,  it  must  have  taken  place  in  the  most 
unstable  and  variable  part  of  the  nervous  system,  the  higher 
cortical  centres  whose  activity  is  accompanied  by  conscious- 
ness. In  other  words,  we  may  be  practically  assured  that 
consciousness  accompanies  learning  only  when  the  learning 
is  so  rapid  as  to  show  that  the  effects  of  previous  experience 
are  recalled  in  the  guise  of  an  idea  or  mental  image  of  some 
sort.  But  does  even  the  most  rapid  learning  possible  assure 
us  of  the  presence  of  an  idea  in  the  mind  of  a  lower  animal  ? 
Where  the  motive,  the  beneficial  or  harmful  consequence  of 
action,  is  very  strong,  may  not  a  single  experience  suffice  to 
modify  action  without  being  revived  in  idea?  Moreover, 


34  The  Animal  Mind 

animals  as  high  in  the  scale  as  dogs  and  cats  learn  to  solve 
problems  analogous  to  that  of  the  combination  lock  so  slowly 
that  we  cannot  infer  the  presence  of  ideas.  Are  we  then  to 
conclude  that  these  animals  are  unconscious,  or  that  there  is 
absolutely  no  reason  for  supposing  them  possessed  of  con- 
sciousness? Yerkes  has  criticised  the  "learning  by  expe- 
rience" criterion  by  pointing  out  that  "  no  organism  .  .  .  has 
thus  far  been  proved  incapable  of  profiting  by  experience." 
It  is  a  question  rather  of  the  rapidity  and  of  the  kind  of  learn- 
ing involved.  "The  fact  that  the  crayfish  needs  a  hundred 
or  more  experiences  for  the  learning  of  a  type  of  reaction  that 
the  frog  would  learn  with  twenty  experiences,  the  dog  with 
five,  say,  and  the  human  subject  with  perhaps  a  single  ex- 
perience, is  indicative  of  the  fundamental  difficulty  in  the  use 
of  this  sign"  (463).  Nagel  has  pointed  out  that  Loeb,  in 
asserting  "associative  memory"  as  the  criterion  of  conscious- 
ness, offers  no  evidence  for  his  statement  (294).  The  fact  is 
that  while  proof  of  the  existence  of  mind  can  be  derived  from 
animal  learning  by  experience  only  if  the  learning  is  very 
rapid,  other  evidence,  equally  valid  on  the  principle  of  anal- 
ogy, makes  it  highly  improbable  that  all  animals  which  learn 
too  slowly  to  evince  the  presence  of  ideas  are  therefore  uncon- 
scious. This  evidence  is  of  a  morphological  character. 

§  7.    Inferring  Mind  from  Structure 

Both  Yerkes  and  Lukas  urge  that  the  resemblance  of  an 
animal's  nervous  system  and  sense-organs  to  those  of  human 
beings  ought  to  be  taken  into  consideration  in  deciding  whether 
the  animal  is  conscious  or  not.  Lukas  suggests  that  the  cri- 
teria of  consciousness  should  be  grouped  under  three  heads : 
morphological,  including  the  structure  of  the  brain  and  sense- 
organs,  physiological,  and  teleological.  Under  the  second 


The  Evidence  of  Mind  35 

rubric  he  maintains  that  "individual  purposiveness "  is  char- 
acteristic of  the  movements  from  which  consciousness  may 
be  inferred;  that  individual  purposiveness  pertains  only  to 
voluntary  acts,  and  that  voluntary  acts  are  acts  "which  are 
preceded  by  the  intention  to  perform  a  definite  movement, 
hence  by  the  idea  of  this  movement."  We  have  reached  the 
same  conclusion  in  the  preceding  paragraph.  The  third  test 
of  the  presence  of  consciousness,  the  teleological  test,  rests 
on  the  consideration:  "What  significance  for  the  organism 
may  be  possessed  by  the  production  of  a  conscious  effect  by 
certain  stimuli  ?  "  (252).  This  test,  however,  being  of  a  purely 
a  priori  character,  would  seem  to  be  distinctly  less  valuable 
than  the  others. 

Yerkes  proposes  "the  following  six  criteria  in  what  seems 
to  me  in  general  the  order  of  increasing  importance.  The 
functional  signs  are  of  greater  value  as  a  rule  than  the  struc- 
tural ;  and  within  each  of  the  categories  the  particular  sign  is 
usually  of  more  value  than  the  general.  In  certain  cases, 
however,  it  might  be  maintained  that  neural  specialization  is 
of  greater  importance  than  modifiability. 
I.  Structural  Criteria. 

1.  General  form  of  organism  (Organization). 

2.  Nervous  system  (Neural  organization). 

3.  Specialization  in  the  nervous  system  (Neural  spe- 

cialization). 
II.    Functional  Criteria. 

1.  General  form  of  reaction  (Discrimination). 

2.  Modifiability  of  reaction  (Docility). 

3.  Variability  of  reaction  (Initiative)"  (463). 

The  terms  "discrimination,"  "docility,"  and  "initiative" 
in  this  connection  are  borrowed  from  Royce's  "Outlines  of 
Psychology  "  (372). 

If  resemblance  of  nervous  and  sense-organ  structure  to  the 


36  The  Animal  Mind 

human  type  is  to  be  taken  along  with  rapid  learning  as  co- 
ordinate evidence  of  consciousness,  it  is  clear  that  here  also 
we  have  to  deal  with  a  matter  of  degree.  The  structure  of 
the  lower  animals  differs  increasingly  from  our  own  as  we  go 
down  the  scale.  At  what  degree  of  difference  shall  we  draw 
the  line  and  say  that  the  animals  above  it  may  be  conscious, 
but  that  those  below  it  cannot  be?  No  one  could  possibly 
establish  such  a  line.  The  truth  of  the  whole  matter  seems  to 
be  this :  We  can  say  neither  what  amount  of  resemblance  in 
structure  to  human  beings,  nor  what  speed  of  learning,  consti- 
tutes a  definite  mark  distinguishing  animals  with  minds  from 
those  without  minds,  unless  we  are  prepared  to  assert  that  only 
animals  which  learn  so  fast  that  they  must  have  memory  ideas 
possess  mind  at  all.  And  this  would  conflict  with  the  argu- 
ment from  structure.  For  example,  there  is  no  good  experi- 
mental evidence  that  cats  possess  ideas,  yet  there  is  enough 
analogy  between  their  nervous  systems  and  our  own  to  make 
it  improbable  that  consciousness,  so  complex  and  highly 
developed  in  us,  is  in  them  wholly  lacking.  We  know  not 
where  consciousness  begins  in  the  animal  world.  We  know 
where  it  surely  resides  —  in  ourselves;  we  know  where  it 
exists  beyond  a  reasonable  doubt  —  in  those  animals  of 
structure  resembling  ours  which  rapidly  adapt  themselves  to 
the  lessons  of  experience.  Beyond  this  point,  for  all  we 
know,  it  may  exist  in  simpler  and  simpler  forms  until  we 
reach  the  very  lowest  of  living  beings. 


CHAPTER  III 
THE  MIND  OF  THE  SIMPLEST  ANIMALS 

§  8.    The  Structure  and  Behavior  of  Amoeba 

WE  have  seen  in  the  last  chapter  that  no  one  can  prove 
the  absence  of  consciousness  in  even  the  simplest  forms  of 
living  beings.  It  is  therefore  perfectly  allowable  to  speculate 
as  to  what  may  be  the  nature  of  such  consciousness,  provided 
that  the  primitive  organisms  concerned  'possess  it.  Per- 
fectly allowable,  yet  also  perfectly  useless,  many  authorities 
would  argue ;  the  remoteness  of  the  creatures  from  ourselves 
in  structure  and  behavior  renders  theorizing  .about  their 
conscious  experience,  which  is  probably  non-existent  and 
certainly  unimaginable  in  any  definite  terms  by  us,  the  idlest 
form  of  mental  exercise. 

Undeniably  the  formation  of  a  positive  notion  regarding 
the  character  arid  content  of  psychic  states  in  the  mind,  say 
of  an  Amoeba,  is  next  door  to  an  impossibility.  Yet  it  may 
not  be  wholly  a  waste  of  time  if  we  spend  a  few  pages  in  the 
attempt  to  discover  wherein  the  simplest  type,  of  mind,  sup- 
posing it  to  be  that  belonging  to  the  simplest  type  of  animal, 
necessarily  differs  from  our  own.  Some  light,  perhaps,  may 
be  cast  upon  the  growth  of  mental  life  -in  complexity  if  we 
try  to  make  clear  to  ourselves  what  primitive  consciousness  is 
not,  though  we  may  not  be  able  to  find  in  our  own  experience 
any  elements  that  shall  properly  represent  what  it  is. 

The  first  need  is  evidently  information  about  the  structure 
and  the  behavior  of  a  primitive  animal.  For  this  purpose  the 

37 


38  The  Animal  Mind 

Amoeba  presents  itself  as  a  good  subject.  Structurally,  it  con- 
sists of  a  single  cell,  as  dp  all  the  Protozoa,  the  lowest  group 
of  animals ;  it  is  so  small  that  it  can  be  studied  only  through 
the  microscope;  its  form,  at  least  that  of  Amceba proteus,  the 
most  typical  species,  is  irregular  and  constantly  changing  in 
locomotion  or  in  response  to  stimulation.  While  the  internal 
substance  of  its  body  shows  a  certain  amount  of  differentiation, 
there  is  no  trace  whatever  of  special  modifications  that  might 
be  supposed  to  serve  for  the  conduction  of  stimuli  to  different 
parts  of  the  body,  and  thus  represent  the  prototype  of  a  ner- 
vous system.  Nor  have  any  structures  been  found  that  could 
conceivably  be  used  for  the  special  reception  of  stimuli ;  that 
is,  there  are  no  sense  organs.  So  far  as  the  anatomy  of  the 
animal  is  concerned,  then,  it  differs  so  widely  from  our  own 
that  we  could  only  conclude  from  it  the  absence  of  all  those 
features  which  our  conscious  experience  involves. 

Turning  from  structure  to  behavior,  we  find  the  external 
activities  of  Amceba,  that  is,  those  not  confined  to  the  inner 
processes  of  its  cell  body,  to  be  superficially,  at  least,  divisible 
into  two  classes :  movements  of  locomotion  and  responses  to 
stimulation.  Amceba,  though  a  water-dwelling  animal,  is 
not  a  free-swimming  one,  but  moves  by  crawling  on  a  solid 
body.  This  method  of  locomotion  involves  in  A  mceba  proteus 
changes  of  form  on  the  animal's  part,  projections,  called  pseu- 
dopodia,  being  sent  out  in  advance  of  the  movement  of  the 
whole  body.  The  protoplasm  of  the  body  shows  in  this  pro- 
cess certain  flowing  movements  which  are  differently  described 
by  different  observers,  and  doubtless  vary  in  different  species : 
thus  Rhumbler  finds  that  the  protoplasmic  currents  move 
backward  along  the  sides  of  the  animal  and  forward  through 
the  middle  in  a  way  quite  comparable  to  the  behavior  of  cur- 
rents in  a  drop  of  any  fluid  where  the  tension  of  the  surface 
is  diminished  in  front,  i.e.,  at  the  point  toward  which  the  drop, 


The  Mind  of  the  Simplest  Animals          39 

in  consequence  of  the  diminished  tension  there,  rolls.  Such 
movements,  Rhumbler  shows,  can  be  reproduced  by  placing, 
say,  a  drop  of  clove  oil  under  the  proper  conditions  of  surface 
tension  (361,  362).  Jennings,  on  the  other  hand,  has  ob- 
served, at  least  in  certain  species  of  Amoeba,  that  the  proto- 
plasmic currents  are  all  forward  in  direction,  the  movement 
being  really  one  of  rolling,  complicated  by  the  attachment  of 
the  lower  part  of  the  body  to  the  solid  object  on  which  the 
animal  crawls.  Mechanical  conditions  of  surface  tension 
would  not  account  for  such  currents  (204,  206,  211).  Del- 
linger,  finally,  rejects  both  the  surface  tension  and  the  "rolling" 
theories,  and  from  a  study  of  side  views  of  the  moving  Amoeba 
concludes  that  progression  occurs  through  the  advancement 
of  the  front  end  freely  through  the  water  and  its  subsequent 
attachment,  the  rest  of  the  body  following  through  active  con- 
traction brought  about  by  a  contractile  substance  (98).  The 
problem  is  of  great  interest  to  the  student  of  vital  phenomena, 
but  its  bearing  on  the  question  of  mind  in  the  Amceba  is  so 
obscure  that  we  need  not  consider  it  further,  but  may  pass  at 
once  to  the  study  of  the  animal's  reactions  to  special  stimula- 
tion. 

These  are,  according  to  Jennings  (206,  211),  the  foremost 
authority  on  the  behavior  of  the  lowest  organisms,  three  in 
number;  namely,  the  negative,  the  positive,  and  the  food- 
taking  reactions.  First,  if  an  Amceba  comes  into  strong  con- 
tact with  a  solid  obstacle  in  its  movements,  or  if  a  solution  of 
different  composition  from  the  water  in  which  it  lives  strikes 
against  it,  or  if  one  side  of  it  is  heated,  the  animal  responds 
by  contracting  the  part  stimulated,  releasing  it  from  the  sub- 
stratum, and  moving  in  another  direction,  usually  one  form- 
ing only  a  small  angle  with  the  preceding  one.  If  the  whole 
of  one  side  or  end  receives  a  strong  stimulus,  if  light  falls  on 
one  side,  or  an  electric  current  is  passed  through  the  water,  the 


The  Animal  Mind 


side  stimulated  —  in  the  case  of  the  electric  current,  the  side 
toward  the  positive  pole  —  contracts  as  a  whole,  and  the 
movement  takes  place  in  the  opposite  direction.  These  phe- 
nomena constitute  the  negative  reaction  (Fig.  i). 

Secondly,  the  reaction  to  solid  bodies  sometimes  takes  a 
positive  form.  In  this  case  a  pseudopodium  is  pushed  for- 
ward in  the  direction  of  the  stimulus,  and  the  animal  moves 

toward  the  solid.  As  the  nega- 
tive reaction  serves  the  purpose 
of  avoiding  obstacles,  so  the 
positive  reaction  is  useful  in 
securing  contact  with  a  support 
on  which  to  creep,  and  with 
food.  It  seems  to  be  given  in 
response  to  weak  mechanical 
stimuli,  stronger  ones  producing 
the  negative  reaction.  No 
chemicals  have  been  found  to 
occasion  it,  but  weak  chemical 
stimulation  very  likely  cooper- 
ates with  mechanical  stimula- 
tion when  the  positive  reaction 
is  given  to  food. 
Thirdly,  there  is  the  food-taking  reaction.  This  consists, 
for  Amceba  proteus,  in  the  pushing  forward  of  a  pseudopodium 
on  either  side  of  the  particle  of  food  that  has  come  into  con- 
tact with  the  animal ;  the  bending  over  of  the  ends  of  the  pseu- 
dopodia  so  as  to  grasp  the  food,  while  "a  thin  sheet  of  pro- 
toplasm" spreads  from  the  upper  surface  of  the  animal  over 
it ;  and  the  final  fusion  of  the  ends  of  the  pseudopodia  and  the 
ends  of  this  sheet,  so  as  to  take  the  food  directly  into  the  ani- 
mal's body.  The  reaction  may  occur  anywhere  on  the  body 
surface,  there  being  no  specialized  mouth.  It  appears  to  be 


FIG.  i.  —  Negative  reaction  of 
Amoeba  to  stimulation  by  a  glass 
rod.  a.  Application  of  the  stim- 
ulus, b.  Change  of  direction 
of  movement.  After  Jennings 

(211). 


The  Mind  of  the  Simplest  Animals         41 

made  only  in  response  to  edible  substances,  hence  there  is 
doubtless  some  chemical  peculiarity  about  the  stimulus  which 
makes  it  effective  (Fig.  2). 

These  three  reactions  make  up,  together  with  the  ordinary 
crawling  locomotion,  the  variety  of  the  Amoeba's  experience 
as  displayed  in  behavior,  with  the  addition  of  a  peculiar  set 
of  movements  occurring  in  the  absence  of  all  mechanical  stimu- 
lation. When  an  Amoeba  is  floating  in  the  water,  through  some 
chance,  unattached  to  any  solid,  "such  a  condition,"  says 


FIG.  2.  —  Food-taking  reaction  of  Amoeba,     i,  2,  3,  4,  successive  stages. 
After  Jennings  (211). 

Jennings,  "is  most  unfavorable  for  its  normal  activities ;  it  can- 
not move  from  place  to  place,  and  has  no  opportunity  to  obtain 
food."  Its  mode  of  getting  out  of  the  difficulty  is  to  send  out 
"long,  slender  pseudopodia  in  all  directions,"  until  "the  body 
may  become  reduced  to  little  more  than  a  meeting  point  for 
these  pseudopodia"  (211,  p.  8).  As  soon  as  one  of  these 
"feelers"  comes  in  contact  with  a  solid,  it  attaches  itself,  and 
the  whole  animal  following  soon  takes  up  its  normal  crawling 
locomotion. 

§  9.     The  Mind  of  Am&ba 

Now  what  light  does  the  behavior  of  Amoeba,  thus  described 
in  its  various  forms  by  Jennings,  throw  upon  the  nature  of  the 


42  The  Animal  Mind 

animal's  possible  consciousness?  The  first  thought  which 
strikes  us  in  this  connection  is  that  the  number  of  different 
sensations  occurring  in  an  Amoeba's  mind,  if  it  has  one,  is 
very  much  smaller  than  the  number  forming  the  constituent  ele- 
ments of  our  own  experience.  We  human  beings  have  the 
power  to  discriminate  several  thousand  different  qualities 
of  color,  brightness,  tone,  noise,  temperature,  pressure,  pain, 
smell,  taste,  and  other  sensation  classes.  Thus  the  content 
of  our  consciousness  is  capable  of  a  great  deal  of  variety.  It 
is  hard  to  see  how  more  than  three  or  four  qualitatively  dif- 
ferent processes  can  enter  into  the  conscious  experience  of  an 
Amoeba.  The  negative  reaction  is  given  to  all  forms  of  strong 
stimulation  alike,  with  the  single  exception  of  food.  We  shall 
in  the  following  chapter  discuss  more  fully  the  nature  of  the 
evidence  that  helps  us  to  conjecture  the  existence  of  different 
sensation  qualities  in  an  animal's  mind ;  but  it  is  clear  that 
where  an  animal  so  simple  in  its  structure  as  the  Amoeba 
makes  no  difference  in  its  reactions  to  various  stimuli,  there 
can  be  no  reason  for  supposing  that  if  it  is  conscious,  it  is 
aware  of  them  as  different.  The  reaction  to  edible  sub- 
stances is,  however,  unlike  that  to  other  stimulations.  The 
peculiarity  of  edible  substances  which  occasions  this  differ- 
ence must  be  a  chemical  one.  In  our  own  case,  the  classes 
of  sensation  which  result  from  the  chemical  peculiarities  of 
food  substances  are  smell  and  taste ;  evidently  to  a  water- 
dwelling  animal  smell  and  taste  would  be  practically  indis- 
tinguishable. We  may  say,  then,  that  supposing  conscious- 
ness to  exist  in  so  primitive  an  animal  as  the  Amceba,  we 
have  evidence  for  the  appearance  in  it  of  a  specific  sensation 
quality  representing  the  chemical  or  food  sense,  and  standing 
for  the  whole  class  of  sensations  resulting  from  our  own 
organs  of  smell  and  taste.  The  significance  of  the  positive 
reaction  is  harder  to  determine.  It  seems  to  be  given  in  re- 


The  Mind  of  the  Simplest  Animals         43 

sponse  not  to  a  special  kind  of  stimulus,  but  to  a  mechanical 
or  food  stimulus  of  slight  intensity.  In  our  own  experience, 
we  do  not  have  stimuli  of  different  intensity  producing  sen- 
sations of  different  quality,  except  in  the  cases  of  temperature 
and  visual  sensations.  We  do,  however,  find  that  varying  the 
strength  of  the  stimulus  will  produce  different  affective  quali- 
ties ;  it  is  a  familiar  fact  that  moderate  intensities  of  stimula- 
tion in  the  human  organism  are  accompanied  by  pleasant- 
ness, and  stronger  intensifies  by  unpleasantness.  The  motor 
effects  of  pleasantness  and  unpleasantness  in  ourselves  are 
opposite  to  each  other  in  character.  Pleasantness  produces 
a  tonic  and  expansive  effect  on  the  body,  unpleasantness  a 
depressive  and  contractive  effect.  In  the  Amoeba,  the  posi- 
tive and  negative  reactions  seem  to  be  opposed.  The  essen- 
tial feature  of  the  negative  reaction  is  the  checking  of  move- 
ment at  the  point  stimulated ;  that  of  the  positive  reaction  is 
the  reaching  out  of  the  point  stimulated  in  the  direction  of  the  « 
stimulus.  This  much  evidence  there  is  for  saying  that  besideTV 
a  possible  food  sensation,  the  Amoeba  may  have  some  dim  K 
awareness  of  affective  qualities  corresponding  to  pleasantness  J  I/ 
and  unpleasantness  in  ourselves.  It  should,  however,  be~~~ 
borne  in  mind  that  wide  differences  must  go  along  with  the 
correspondence.  In  us,  pleasantness  brings  a  thrill,  a  "bodily 
resonance,"  due  to  its  tonic  effect  upon  the  circulation,  breath- 
ing, and  muscles ;  unpleasantness  has  also  its  accompaniment 
of  vague  organic  sensation,  without  which  we  can  hardly  con- 
ceive what  it  would  be  like.  In  an  Amoeba,  it  is  clear  that 
this  aspect,  as  found  in  human  consciousness,  must  be  wholly 
lacking.  Again,  in  the  human  mind  pleasantness  "and  un- 
pleasantness are  connected  with  various  sensation  qualities 
or  complexes;  we  are  pleased  or  displeased  usually  "at" 
something  definite.  The  vagueness  of  the  affective  qualities 
in  an  Amoeba's  consciousness  can  only  be  remotely  suggested 


44  The  Animal  Mind 

by  our  own  vague,  diffused  sense  of  bodily  well-being  or  ill- 
being  ;  and  this  is  undoubtedly  given  its  coloring  in  our  case 
by  the  structure  and  functioning  of  our  internal  organs. 

As  for  the  peculiar  behavior  of  an  Amoeba  suspended  in 
the  water  and  deprived  of  solid  support,  the  stimulus  for  this 
must  lie  within  the  cell  body  itself.  If  any  consciousness 
accompanies  it,  then  the  nearest  human  analogy  to  such 
consciousness  is  to  be  found  in  organic  sensations,  and  these, 
as  has  just  been  said,  must  necessarily  be  in  the  human  mind 
wholly  different  in  quality  from  anything  to  be  found  in  an 
animal  whose  structure  is  as  simple  as  the  Amoeba's. 

A  consequence  of  this  lack  of  qualitative  variety  in  the  sense 
experiences  of  an  Amoeba  is  a  lack  of  what  we  may  call  com- 
plexity of  structure  in  that  experience.  The  number  of 
stimulus  differences  which  are  in  the  human  mind  represented 
by  differences  in  the  quality  of  sensations  is  so  great  that  at 
any  given  moment  our  consciousness  of  the  external  world  is 
analyzable  into  a  large  number  of  qualitatively  different  sen- 
sations. At  the  present  instant  the  reader's  consciousness 
"contains,"  apart  from  the  revived  effects  of  previous  stimu- 
lation, many  distinguishable  sensation  elements,  visual,  audi- 
tactile,  organic,  and  so  on.  The  Amoeba's  conscious- 
.ess,  if  it  possesses  one,  must  have  a  structure  inconceivably 
impler  than  that  of  any  moment  of  our  own  experience. 

A  second  point  in  which  the  mind  of  an  Amceba  must,  if  it 
exists,  differ  from  that  of  a  human  being,  consists  in  its  entire 
lack  of  mental  imagery  of  any  sort.  Not  only  has  the  Amceba 

t  three  or  four  qualitatively  different  elements  in  its  expe- 
rience, but  none  of  these  qualities  can  be  remembered  or 
revived  in  the  absence  of  external  stimulation.  How  may 
we  be  sure  of  this  ?  If  our  primitive  animal  could  revive  its 
experiences  in  the  form  of  memory  images,  it  would  give  some 
evidence  of  the  influence  of  memory  in  its  behavior.  Indeed, 


The  Mind  of  the  Simplest  Animals         45 

as  we  shall  learn,  it  is  possible,  in  all  probability,  for  an  ani- 
mal's conduct  to  be  influenced  by  its  past  experience  even 
though  the  animal  be  incapable  of  reviving  that  experience  in 
the  form  of  a  memory  image.  Therefore,  if  we  find  no  evi- 
dence that  the  Amoeba  learns,  or  modifies  its  behavior  as  the 
result  of  past  stimulation,  we  may  conclude  a  fortiori  that  it 
does  not  have  memory  images. 

Now  it  would  be  stating  the  case  too  strongly  to  say  that 
past  stimulation  does  not  affect  the  behavior  of  Amoeba  at  all. 
In  the  first  place,  this  animal  shows,  in  common  with  all  other 
animals,  the  power  of  " getting  used"  to  certain  forms  of 
stimulation,  so  that  on  long' continuance  they  cease  to  provoke 
reaction.  "Thus,"  Jennings  says,  "Amoebae  react  negatively 
to  tap  water  or  to  water  from  a  foreign  culture,  but  after  trans- 
ference to  such  water  they  behave  normally"  (211,  p.  20). 
Such  cessation  of  reaction  occurs  when  the  continued  stimulus 
is  not  harmful.  In  a  sense,  it  may  be  called  an  effect  of  ex- 
perience; but  there  is  clearly  no  reason  for  supposing  that 
it  involves  the  revival  of  experience  in  the  form  of  an  idea  or 
image.  We  have  parallel  phenomena  in  our  own  mental 
life.  A  continued  stimulus  ceases  to  be  "noticed,"  but  the 
process  involves  rather  the  disappearance  of  consciousness 
than  the  appearance  of  a  memory  image.  Jennings,  how- 
ever, is  inclined  to  think  that  preceding  stimulation  may 
modify  the  Amoeba's  behavior  in  a  way  more  nearly  suggest- 
ing memory  in  a  higher  type  of  mind.  He  describes  an 
interesting  observation  to  illustrate  this.  A  large  Amoeba, 
c,  had  swallowed  a  smaller  one,  b,  but  had  left  a  small  canal 
open,  through  which  the  swallowed  one  made  efforts  to  escape, 
which  were  several  times  foiled  by  movements  on  the  part  of 
the  large  Amoeba  toward  surrounding  it  again.  Finally  it 
succeeded  in  getting  completely  out,  whereupon  the  large 
Amoeba  "reversed  its  course,  overtook  b,  engulfed  it  com- 


46  The  Animal  Mind 

pletely  again,  and  started  away."  The  small  Amoeba  con- 
tracted into  a  ball  and  remained  quiet  until  through  the  move- 
ments of  the  large  one  there  chanced  to  be  but  a  thin  layer 
of  protoplasm  covering  it.  This  it  rapidly  pushed  through, 
escaped  completely,  and  was  not  pursued  by  the  large  Amoeba 
(211,  pp.  17-18),  (Fig.  3). 

Of  this  performance  Jennings  says,  "It  is  difficult  to  con- 
ceive each  phase  of  action  of  the  pursuer  to  be  completely 
determined  by  a  simple  present  stimulus.  For  example  .  .  . 
after  Amoeba  b  has  escaped  completely  and  is  quite  separate 
from  Amoeba  c,  the  latter  reverses  its  course  and  recaptures 
b.  What  determines  the  behavior  of  c  at  this  point  ?  If  we 
can  imagine  all  the  external  physical  and  chemical  conditions 
to  remain  the  same,  with  the  two  Amoebae  in  the  same  relative 
positions,  but  suppose  at  the  same  time  that  Amoeba  c  has 
never  had  the  experience  of  possessing  b}  —  would  its  action 
be  the  same?  Would  it  reverse  its  movement,  take  in  b, 
then  return  on  its  former  course  ?  One  who  sees  the  behavior 
as  it  occurs  can  hardly  resist  the  conviction  that  the  action  at 
this  point  is  partly  determined  by  the  change  in  c  due  to  the 
former  possession  of  b,  so  that  the  behavior  is  not  purely 
reflex"  (211,  p.  24). 

If  it  is  true  that  an  Amoeba  which  had  not  just  uhad  the 
experience  of  possessing  6"  would  not  have  reversed  its  move- 
ment and  gone  after  b  when  the  latter  escaped,  still  we  cannot 
think  it  possible  that  c's  movements  in  so  doing  were  guided 
by  a  memory  image  of  b.  It  may  be  supposed  that  the  recent 
stimulation  of  contact  with  b  had  left  a  part  of  c's  protoplasm 
in  a  condition  of  heightened  excitability,  so  that  the  weak 
stimulus  offered  perhaps  by  slight  water  disturbances  due 
to  b's  movements  after  escaping  produced  a  positive  reaction, 
although  under  other  circumstances  no  reaction  would  have 
been  possible.  In  any  case,  there  is  no  evidence  that  Amoeba's 


48  The  Animal  Mind 

behavior  is  influenced  by  stimulation  occurring  earlier  than 
the  moments  just  preceding  action ;  no  proof  of  the  revival 
of  a  process  whose  original  effects  have  had  time  to  die  out ; 
and  it  is  upon  such  revival  that  the  memory  images  which 
play  so  much  part  in  our  own  conscious  life  depend. 

Let  us  consider  for  a  moment  some  of  the  results  of  the 
absence  of  this  kind  of  material  in  the  possible  mental  pro- 
cesses of  Amoeba.  In  the  first  place,  such  a  lack  profoundly 
affects  the  character  of  the  experiences  which  the  animal 
might  be  supposed  to  receive  through  external  stimulation. 
If  we  call  the  possible  conscious  effect  of  a  mechanical  stimu- 
lus upon  the  Amoeba  a  touch  sensation,  the  term  suggests, 
naturally,  such  sensations  as  we  ourselves  experience  them. 
In  normal  human  beings  touch  sensations  are  accompanied 
by  visual  suggestions,  more  or  less  clear,  of  course,  according 
to  the  visualizing  powers  of  the  individual,  but  always  present 
in  some  degree.  Fancy,  for  example,  one  of  us  entering  a 
room  in  the  dark  and  groping  about  among  the  furniture. 
How  constantly  visual  associations  are  brought  into  play ! 
Not  once  is  a  mere  touch  impression  apprehended  without 
being  translated  into  visual  terms ;  the  forms  and  positions  of 
the  articles  encountered  are  thought  of  immediately  as  they 
would  appear  if  the  room  were  lighted.  The  difficulty  we 
have  in  thinking  of  a  touch  sensation  with  no  visual  associa- 
tions illustrates  the  difference  between  our  sense  experience 
and  that  of  an  animal  incapable  of  recalling  images  of  past 
sensations. 

It  is  equally  obvious  that  in  the  absence  of  memory  ideas, 
not  only  must  the  Amoeba  lack  processes  of  imagination  and 
reasoning,  but  there  can  be  nothing  like  the  continuous  self- 
consciousness  of  a  human  being,  the  "  sense  "  of  personal  iden- 
tity, which  depends  upon  the  power  to  revive  past  experiences. 
It  is  even  possible  that  the  "stream  of  consciousness"  for  an 


The  Mind  of  the  Simplest  Animals         49 

Amoeba  may  not  be  a  continuous  stream  at  all.  Since  its 
sensitiveness  to  changes  in  its  environment  is  less  developed 
than  that  of  a  human  being,  and  there  are  no  trains  of  ideas 
to  fill  up  possible  intervals  between  the  occurrences  of  out- 
side stimulation,  the  Amoeba's  conscious  experience  may  be^v 
rather  a  series  of  " flashes"  than  a  steady  stream.  And  for  7 
the  Amoeba,  again,  we  must  remember  that  even  such  a  series 
would  not  exist  as  such ;  the  perception  of  a  series  would  involve 
the  revival  of  its  past  members.  Each  moment  of  conscious- 
ness is  as  if  there  were  no  world  beyond,  before,  and  after  it. 
Another  consequence  of  that  simplicity  of  structure  which 
results  both  from  the  rudimentary  powers  of  sensory  discrimi- 
nation and  from  the  absence  of  memory  ideas  in  the  Amoeba's 
mind  is  that  there  can  be  no  distinction,  within  a  given  mental 
process,  between  that  which  is  attended  to  and  that  which  is 
not  attended  to,  between  the  focus  and  the  margin  of  conscious- 
ness. Given  a  consciousness  which  at  a  certain  moment  is 
composed  of  the  qualitatively  different  elements  A,  B,  C,  and 
D,  we  can  understand  what  is  meant  by  saying  that  A  is 
attended  to,  is  in  the  foreground  of  attention,  while  B,  C, 
and  D  remain  in  the  background.  But  given,  on  the  other 
hand,  a  creature  whose  conscious  content  at  a  certain  time 
consists  wholly  of  the  qualitatively  simple  experience  A,  it 
is  evident  that  attention  and  inattention  are  meaningless 
terms.  Different  moments  of  its  consciousness  may  differ 
in  intensity;  but  attention,  involving,  as  it  does,  clearness 
rather  than  intensity,  arises  only  when  mental  states  have 
become  complex  and  possess  detail  and  variety  within  their 
structure. 

§  10.    The  Structure  and  Behavior  of  Paramecium 

Although  Amoeba  represents  in  structure  the  simplest  form 
of  animal  life,  its  behavior  in  response  to  stimulation  is  rather 


50  The  Animal  Mind 

more  complex  than  that  of  some  other  members  of  the  type 
Protozoa.  There  is  a  large  group  of  single-celled  animals 
called  Ciliata,  from  the  fact  that  their  bodies  are  covered  with 
little  hairlike  protoplasmic  filaments  or  cilia  which  serve  as 
organs  of  locomotion  by  acting  like  tiny  oars.  A  common 
representative  of  the  group  is  Paramecium.  The  structure  of 
this  animal  is  distinctly  more  specialized  than  that  of  Amoeba. 
Not  only  are  the  cilia  modified  locomotory  structures,  but 
there  is  a  definite  region  for  food -taking.  A  groove  extends 
obliquely  down  one  side  of  the  body,  terminating  at  its 
lower  end  in  a  mouth.  The  cilia  along  this  oral  groove 
beat  with  especial  vigor  and  create  currents  which  sweep 
food  particles  to  the  mouth.  Paramecium  swims  rapidly 
through  the  water  with  a  spiral  motion  of  its  body,  due 
to  the  facts  that  the  aboral  cilia  beat  more  strongly  than 
the  rest,  and  that  the  animal  compensates  for  the  turning 
thus  occasioned  by  turning  on  its  long  axis.  Its  reactions 
to  stimulation  Jennings  has  shown  to  be  only  two  in 
number.  First,  there  is  a  very  definite  avoiding  or  negative 
reaction.  This  is  given  in  response  to  decided  mechanical 
stimulation  at  the  anterior  end,  as  when  the  animal 
swims  rapidly  against  an  obstacle,  and  also  in  response 
to  chemical  stimulation,  to  strong  ultra-violet  rays  (167),  and 
to  temperatures  above  or  below  a  certain  middle  region  called 
in  this  case,  as  in  analogous  cases  with  other  animals,  the 
optimum.  For  Paramecium  it  lies  between  24°  and  28°  C. 
The  negative  reaction  consists,  according  to  Jennings,  of  the 
following  process :  the  animal  darts  backward,  reversing  the 
beat  of  its  cilia,  turns  toward  the  aboral  side  (that  opposite  to 
the  oral  groove)  by  increasing  the  beat  of  the  oral  cilia  and 
lessening  the  compensating  rotation,  and  continues  on  a 
forward  course  that  is  now  at  an  angle  with  its  former  line  of 
motion.  If  this  new  course  carries  it  clear  of  the  stimulus, 


The  Mind  of  the  Simplest  Animals         51 

it  continues  on  its  way ;  if  not,  repeated  contact  with  the  stim- 
ulus causes  a  second  reaction,  the  Paramecium  always  turning 
in  the  same  direction,  so  that  ultimately  it  avoids  the  source 
of  stimulation  (194,  211)  (Fig.  4).  Differing  strengths  of 
stimulus  produce  the  reaction  with  different  degrees  of  vio- 
lence. When  a  very  strong  stimulus  is  encountered,  the 
animals  "respond  first  by  swimming  a  long  way  backward, 
thus  removing  themselves  as  far  as  possible  from  the  source 
of  stimulation.  Then  they  turn  directly  toward  the  aboral 


FIG.  4.  —  Negative  reaction  of  Paramecium.    A  is  the  source  of  stimulation. 
1-6  are  the  successive  positions  of  the  animal.    After  Jennings  (211). 

side,  —  the  rotation  on  the  long  axis  completely  ceasing. 
In  this  way  the  animal  may  turn  directly  away  from  the  drop 
[the  stimulus]  and  retrace  its  course"  (211,  p.  50).  On  the 
other  hand,  when  the  stimulus  is  very  weak  the  reaction  may 
be  reduced  to  the  following  form:  the  Paramecium  " merely 
stops,  or  progresses  more  slowly,  and  begins  to  swing  its 
anterior  end  about  in  a  circle."  As  long  as  it  does  not  thus 
get  out  of  range  of  the  stimulus,  the  movement  is  continued. 
"When  the  anterior  end  is  finally  pointed  in  a  direction  from 
which  no  more  of  the  stimulating  agent  comes,  the  Paramecium 
swims  forward  "  (211,  p.  51).  Evidently,  however,  these  are 


52  The  Animal  Mind 

but  differing  degrees  of  a  reaction  whose  essential  features  are 
the  same. 

While  Paramecium  definitely  avoids  by  means  of  this  neg- 
ative reaction  certain  chemicals  introduced  into  the  water,  it 
shows  a  tendency  to  collect  in  the  neighborhood  of  others. 
Such  is  the  case  with  weak  acids,  with  a  bubble  of  oxygen  if 
air  has  been  long  excluded  from  the  slide,  and  with  carbon 
dioxide,  which  in  water  of  course  produces  acid  (214).  Jen- 
nings pointed  out  that  the  inclination  of  Paramecium  to 
gather  in  groups  is  very  likely  due  to  the  attraction  for  them 
of  the  carbon  dioxide  which  they  excrete.  But  he  has  also 
shown  that  this  "attraction"  to  certain  chemicals  does  not 
mean  the  presence  of  a  special  positive  reaction.  The  fact 
is  that  when  the  animals  collect  in  a  drop  of  weak  acid,  for 
example,  they  are  not  drawn  .toward  the  acid.  They  simply 
happen,  in  their  ordinary  movements,  to  swim  into  it,  and  on 
entering  it  show  no  disturbance  whatever.  But  when  they 
come  to  the  edge  of  the  drop  on  their  way  out,  they  give  the 
negative  reaction  to  the  surrounding  water.  In  this  way  they 
are,  as  it  were,  trapped  within  the  drop. 

The  nearest  analogue  to  a  positive  reaction  in  Paramecium 
consists  in  the  fact  that  sometimes,  when  they  come  into 
contact  with  a  solid,  instead  of  darting  backward,  the  animals 
merely  cease  moving,  and  extending  stiffly  the  cilia  which 
touch  the  object,  remain  at  rest  (Fig.  5).  The  utility  of 
this  behavior  is  that  around  decaying  vegetable  matter,  the 
kind  of  solid  oftenest  found  in  the  animal's  ordinary  environ- 
ment, there  is  apt  to  be  a  supply  of  food  in  the  way  of  bacteria ; 
it  is  a  good  anchorage.  What  characteristics  of  the  stimulus 
determine  that  this  "contact  reaction,"  rather  than  the 
negative  reaction,  shall  be  given?  Does  weak  mechanical 
stimulation  occasion  it,  as  happens  with  Amoeba's  positive 
reaction  ?  Evidence  in  favor  of  this  is  offered  by  the  fact  that 


The  Mind  of  the  Simplest  Animals         53 


the  contact  reaction  is  more  likely  to  occur  if  the  animal 
comes  against  the  solid  when  swimming  rather  slowly.  Jen- 
nings reports  also  that  individuals  vary.  "  Often  all  the  indi- 
viduals in  a  culture  are  thus  inclined  to  come  to  rest,  while 
in  another  culture  all  remain  free-swimming, 
and  give  the  avoiding  reaction  whenever  they 
come  in  contact  with  a  solid  "  (211,  p.  60).  This 
would  suggest  that  some  individuals  are  in  a  state 
of  greater  excitability  than  others,  so  that  a  given 
stimulus  acts  more  strongly  upon  them.  On  the 
other  hand,  there  is  a  possibility  that  qualitative 
as  well  as  intensive  differences  in  the  stimulus 
are  responsible  for  the  contrasting  reactions. 
"In  general,"  says  Jennings,  Paramecium 
"shows  a  tendency  to  come  to  rest  against  loose 
or  fibrous  material;  in  other  words,  it  reacts 
thus  to  material  with  which  it  can  come  in 
contact  at  two  or  more  parts  of  the  body  at  once. 
To  smooth,  hard  materials,  such  as  glass,  it  is  much  less 
likely  to  react  in  this  manner  "  (211,  p.  *6i).  Perhaps,  then, 
the  spatial  distribution  of  the  stimulus  over  several  points 
of  the  body  surface  increases  the  probability  of  a  contact 
rather  than  an  avoiding  reaction. 

Certain  other  forms  of  behavior  in  Paramecium  involve 
the  taking  up  of  a  definite  position  with  reference  to  some 
constant  stimulus,  and  are  therefore  termed  by  Jennings 
"orienting  reactions."  In  the  first  place,  if  there  is  a  current 
in  the  water,  the  animals  will  head  up-stream.  Jennings 
explains  this  as  due  to  the  giving  of  avoiding  reactions  in 
response  to  the  disturbing  effects  of  the  current  on  the  cilia 
until,  with  the  Paramecium's  head  up-stream,  the  current  no 
longer  tends  to  reverse  the  cilia.  Analogous  reaction  is  given 
to  gravity ;  the  animals  direct  their  heads  upward,  and  swim 


FIG.  5.— 
Positive 
thigmotaxis 
in  Parame- 
cium.   After 
Jennings 
(211). 


54  The  Animal  Mind 

in  that  direction.  The  cause  of  this  has  been  the  subject  of 
some  dispute,  which  we  shall  discuss  in  a  later  chapter;  but 
the  response  to  gravity  seems  in  any  case  not  to  involve  a  new 
form  of  reaction.  Further,  Paramecium  reacts  to  the  cen- 
trifugal force  produced  by  whirling  a  horizontal  tube  around 
a  vertical  axis  just  as  it  does  to  gravity ;  that  is,  it  orients  itself 
in  such  a  way  as  to  swim  toward  the  axis,  in  the  opposite 
direction  to  the  pull  of  the  force  (211,  p.  78). 

To  an  electric  current  the  response  of  Paramecium  is 
more  complicated.  When  the  current  is  weak  the  animals 
move  toward  the  cathode.  This  appears  to  be  caused  simply 
by  the  giving  of  the  negative  reaction  so  long  as  the  front  end 
of  the  animal  is  turned  toward  the  anode,  and  is  thus  being 
stimulated.  But  if  the  current  is  made  gradually  stronger, 
the  movement  toward  the  cathode  grows  slower  and  finally 
stops.  Further  increase  in  the  intensity  of  the  current  causes 
the  animal  to  swim  backward  toward  the  anode,  and  finally 
to  burst  into  pieces.  This  reversal  of  movement  Jennings 
has  found  to  be  due  to  the  fact  that  the  cilia  nearest  the  cathode 
have  their  direction  reversed ;  as  the  current  is  made  stronger, 
this  effect  is  increased,  until  finally  it  balances  and  prevails 
over  the  beat  of  the  forward  cilia  (21 1,  pp.  82  ff.).  The  animal's 
movements  are  thus  really  discoordinated  by  the  action  of 
the  strong  current.  The  effect  seems  a  pathological  one,  and 
probably  need  not  be  taken  into  account  in  considering  the 
normal  life  of  the  infusorian;  as  Jennings  says,  "The  re- 
action to  electricity  is  purely  a  laboratory  product"  (211, 

p.  168). 

§  ii.    The  Mind  of  Paramecium 

If  we  now  compare  the  behavior  of  Paramecium  with  that 
of  Amoeba  in  order  to  draw  conclusions  with  regard  to  the 
possible  consciousness  of  the  former,  we  find  that  although  the 
mechanism  of  reaction  is  decidedly  more  complicated  in  Para- 


The  Mind  of  the  Simplest  Animals         55 

mecium  than  in  Amoeba,  there  is  rather  less  possibility  of 
variety  in  the  conscious  experience  of  the  ciliated  protozoon. 
The  reversal  of  cilia,  the  rolling  toward  the  aboral  side, 
form  a  more  elaborated  and  specialized  mode  of  withdrawal 
than  does  the  simple  checking  of  protoplasmic  flow  at  one 
region  of  the  body.  But,  supposing  Paramecium  to  be  con- 
scious, the  significance  for  its  consciousness  of  the  negative 
or  avoiding  reaction  is  even  less  clear  than  that  of  the  corre- 
sponding behavior  in  Amoeba.  The  negative  reaction  in 
Amoeba  is  contrasted  on  the  one  hand  with  a  positive  reaction, 
opposite  in  character,  given  to  stimuli  of  less  intensity,  so  that 
the  opposition  of  unpleasantness  and  pleasantness  in  the 
human  mind  is  clearly  suggested ;  and  on  the  other  hand  with 
a  food-taking  reaction,  given  to  edible  substances,  and  hinting 
at  a  differentiation  corresponding  to  that  between  touch  and 
taste  in  our  own  experience.  The  only  approach  to  a  positive 
reaction  in  Paramecium  is  the  coming  to  rest  in  contact  with 
solids,  and  this  does  not  present  any  very  striking  analogy 
with  such  expressions  of  pleasantness  as  we  are  acquainted 
with.  Further,  Paramecium  has  no  special  food-taking 
reaction  at  all.  The  fact  seems  to  be  that  its  greater  speed  of 
motion  has  developed  its  negative  reaction  at  the  expense  of 
the  others.  It  does  not  need  to  reach  out  in  a  typical  positive 
reaction,  for  its  rapid  dashing  through  the  water  greatly 
increases  its  natural  chance  of  getting  food.  The  whirling 
of  the  oral  cilia  brings  it  edible  as  well  as  inedible  substances. 
It  does,  however,  much  need  in  its  headlong  career  a  means 
of  avoiding  the  dangers  into  which  it  may  rush,  and  so  we 
find  the  very  definite  and  well-adapted  negative  reaction 
dominating  the  field.  If,  then,  the  mind  of  an  Amoeba  is 
thought  of  as  capable  of  three  or  four  qualitatively  different 
experiences,  that  of  a  Paramecium  must  be  even  less 
favored. 


56  The  Animal  Mind 

>  Is  there  any  evidence  of  the  presence  in  Paramecium  of 
JL&ie  revival  of  past  experiences  in  any  form  ?  The  answer  to 
\this  question  must  be  negative,  as  in  the  case  of  Amoeba. 
Immediately  preceding  stimulation  does  have  some  effect 
upon  the  response  to  present  stimulation,  but  these  effects 
are  all  of  such  a  character  as  to  suggest  rather  the  disap- 
pearance of  possible  consciousness  than  the  recall  of  a  memory 
image.  Jennings  mentions  several  instances.  "If  a  Para- 
mecium is  subjected  to  a  strong  induction  shock,  it  fails  for 
some  time  thereafter  to  react  to  weak  shocks,  though  at  the 
beginning  it  reacted  to  these"  (211,  p.  100).  Such  a  result 
is  due  probably  to  fatigue.  "Paramecia  which  have  been 
living  at  the  usual  temperatures  show  a  temperature  opti- 
mum of  about  24  to  28  degrees ;  if  they  are  kept  for  some 
hours  at  a  temperature  from  36  to  38  degrees,  the  optimum 
rises  to  30  or  32  degrees.  A  change  in  the  individuals  in- 
duced in  this  way  is  commonly  spoken  of  as  acclimatiza- 
tion "  (211,  p.  101).  Further,  "if  a  bit  of  filter  paper  is  placed 
in  a  preparation  of  Paramecia,  the  following  behavior  may 
often  be  observed.  An  individual  swims  against  it,  gives  the 
avoiding  reaction  in  a  slightly  marked  way,  swimming  back- 
ward a  little;  then  it  swims  forward  again,  jerks  back  a 
shorter  distance,  then  settles  against  the  paper  and  remains. 
After  remaining  a  few  seconds,  it  may  move  to  another 
position,  still  remaining  in  contact  with  the  paper.  Then  it 
may  leave  the  paper  and  go  on  its  way  "(211,  p.  101).  Be- 
havior of  this  type,  where  a  stimulus  at  first  occasions  the 
negative  reaction,  but  on  immediate  repetition  ceases  to  do 
so,  we  shall  find  very  common  among  the  lower  forms  of 
animals;  it  suggests  simply  that  the  stimulus  acts  less  and 
less  strongly  on  repetition,  not  that  the  effects  of  its  earlier 
application  are  consciously  recalled. 

Jennings's  work  on  other  ciliate  Protozoa,  as  well  as  on  the 


The  Mind  of  the  Simplest  Animals         57 

group  known  as  Flagellata,  the  members  of  which  have  in 
place  of  cilia  a  long  whiplike  protoplasmic  filament,  and 
move  by  lashing  it  to  and  fro  in  the  water,  indicates  that  in  all 
of  them  the  negative  reaction  is  the  principal  feature  of  be- 
havior (199,  21 i),  and  that  if  any  of  them  possess  minds,  those 
minds  are  of  quite  as  rudimentary  a  type  as  that  of  Amoeba, 
and  very  likely  rendered  even  more  so,  as  far  as  qualitative 
variety  of  experience  goes,  by  the  predominance  of  the  neg- 
ative reaction  due  to  greater  speed  of  motion. 

§  12.    Definitions  of  Tropisms 

Before  passing  to  the  study  of  higher  forms  of  animal  life, 
we  may  note  the  meaning  of  a  few  technical  terms  used  in 
describing  the  behavior  of  simple  animals  especially.  The 
direct  motor  response  of  an  animal  to  an  external  stimulus  is 
known  as  a  tropism,  from  the  Greek  word  meaning  "to 
turn."  Various  prefixes  are  attached  to  this  term  to  indicate 
the  nature  of  the  stimulus  concerned;  thus  phototropism 
means  the  reaction  of  an  animal  to  light;  chromotropism, 
reaction  to  color ;  thigmotropism,  reaction  to  contact ;  chemo- 
tropism,  reaction  to  chemical  stimulation;  rheofropism, 
reaction  to  currents ;  geotropism,  reaction  to  gravity ;  electro- 
tropism,  reaction  to  the  electric  current;  anemotropism, 
reaction  (e.g.,  in  winged  insects)  to  wind.  Some  writers 
have  used  instead  of  tr'opism  the  word  taxis,  from  the  Greek 
word  meaning  "to  arrange,"  speaking  of  chemotaxis,  thigmo- 
taxis,  and  so  on.  Phototaxis  has,  as  we  shall  see,  a  rather 
special  significance  distinct  from  phototropism.  When  an 
animal  gives  a  positive  reaction  in  response  to  a  stimulus, 
it  is  said  to  be  positively  chemotropic,  or  phototropic,  as  the 
case  may  be ;  when  its  reaction  is  negative,  it  is  called  nega- 
tively chemotropic,  phototropic,  and  so  on. 


CHAPTER    IV 
SENSORY  DISCRIMINATION:   METHODS  OF  INVESTIGATION 

§  13.    Preliminary  Considerations 

ONE  of  the  most  important  points  in  which  the  human 
mind  differs  from  the  mind  of  the  lowest  animal  forms  con- 
sists, we  have  seen,  in  the  enormously  greater  number  of 
different  sensations  which  enter  into  human  experience,  as 
compared  with  the  small  number  of  sensory  discriminations 
possible  to  the  simpler  animals.  Much  of  the  experimental 
work  that  has  been  done  on  animals  has  been  directed 
toward  discovering  what  discriminations  they  make  among 
the  stimuli  acting  upon  them,  and  to  the  results  of  this  work 
we  shall  give  our  attention  in  the  next  chapters.  But  first 
we  ought  to  get  a  clearer  idea  of  just  what  kind  of  evidence 
is  needed  to  indicate  the  existence  of  a  variety  of  sensations 
in  an  animal's  mind. 

At  the  outset,  we  must  remind  ourselves  that,  in  the  ab- 
sence of  any  satisfactory  proof  that  the  lower  animal  forms 
have  minds  at  all,  and  the  equal  absence  of  any  proof  that  they 
have  not,  all  our  conclusions  about  the  number  and  kind  of 
their  possible  sensations  must  remain  subject  to  the  proviso 
that  they  possess  consciousness.  Further,  a  point  that  was 
mentioned  on  page  24  must  again  be  emphasized.  No  evi- 
dence of  discrimination  between  two  stimuli  on  an  animal's 
part  can  do  more  than  show  us  that  for  the  animal  they  are 
different;  just  what  the  quality  of  the  sensation  resulting 
from  each  may  be,  whether  it  is  identical  with  any  sensation 

58 


Sensory  Discrimination  59 

quality  entering  into  our  own  experience,  we  cannot  say. 
The  light  rays  which  to  us  are  red  and  blue  may  for  an 
animal's  consciousness  also  differ  from  each  other,  and  yet 
if  our  experience  could  be  exchanged  for  the  animal's,  we 
might  find  in  the  latter  nothing  like  red  and  blue  as  we  know 
them. 

Thus  much  being  premised,  what  sort  of  evidence  can 
be  obtained  that  an  animal  does  discriminate  between  two 
stimuli?  Again,  as  in  considering  the  evidence  for  the 
existence  of  consciousness  in  general,  there  is  an  argument 
from  structure  and  an  argument  from  behavior. 

§  14.   Structure  as  Evidence  of  Discrimination 

The  argument  from  structure  consists  primarily  in  the  fact 
that  an  animal  possesses  sense  organs  recognizably  like 
our  own.  If  a  creature  has  an  organ  suggesting  strongly  the 
construction  of  the  human  cochlea,  or  an  organ  with  a  lens 
and  a  membrane  composed  of  rods  and  cones,  it  is  highly 
probable  that  auditory  stimuli  in  the  one  case  and  light  in  the 
other  produce  specific  sensations.  This  argument  from  the 
morphology  of  sense  organs. is,  however,  limited  in  two  ways. 
First,  it  is  only  a  small  part  of  the  animal  world  whose  sense 
organs  resemble  ours  closely  enough  to  make  the  analogy 
safe.  And  secondly,  we  do  not  after  all  know  very  much 
about  the  relation  of  our  own  sense-organ  structure  to  function. 
We  know,  for  example,  that  our  own  organ  with  a  lens  and 
retina  gives  us  visual  sensations,  but  we  cannot  say  with 
certainty  which  structures  in  the  retina  furnish  brightness 
sensations  and  which  color  sensations,  nor  do  we  know  any- 
thing about  the  retinal  structures  that  underlie  different 
qualities  of  color  sensations.  We  can  say  that  sensations 
of  hearing  come  from  the  ear,  but  no  one  can  tell  us  how 
to  judge  from  the  structure  of  the  ear  what  range  and 


60  The  Animal  Mind 

fineness  of  pitch  discriminations  exist  in  its  possessor's 
mind.  No  investigator  has  yet  succeeded  in  relating  the 
different  qualities  of  smell  and  taste  to  differences  in  the 
end  organs. 

4  §  15.    Behavior  as  Evidence  of  Discrimination 

The  argument  from  behavior  is  as  follows :  If  an  animal 
reacts  in  a  different  way  to  two  qualitatively  unlike  stimuli, 
then,  providing  that  it  is  conscious  at  all,  it  may  be  supposed 
to  receive  qualitatively  unlike  sensations  from  them.  If  it 
always  reacts  in  the  same  way  to  both,  then  both  may  be  sup- 
posed to  be  accompanied  by  the  same  sensation  quality. 
Obviously  these  statements  need  further  discussion.  For 
one  thing,  it  may  be  urged  that  in  our  own  case  the  same 
external  reaction  is  often  made  to  stimuli  that  are  nevertheless 
consciously  discriminated.  A  man  may  eat  with  relish  and 
without  observable  difference  in  behavior,  for  example,  foods 
that  yet  give  him  perfectly  distinguishable  smell  and  taste 
sensations.  Precisely  this  objection  holds  against  a  method 
of  experimentation,  formerly  a  good  deal  used,  which  may 
be  called  the  Preference  Method  of  testing  discrimination. 
Vitus  Graber,  for  instance,  attempted  to  find  whether  ani- 
mals belonging  to  a  variety  of  species  could  discriminate 
colors,  by  offering  them  the  choice  of  two  compartments 
illuminated  each  with  a  different  color.  Clearly,  if  the  ani- 
mals chose  one  compartment  as  often  as  the  other,  it  would 
be  rash  to  conclude  that  the  two  lights  produced  for  them 
indistinguishable  sensation  qualities.  There  might  simply 
be  the  absence  of  any  preference,  along  with  perfect  dis- 
crimination. The  fact  is  that  in  all  experiments  upon 
animals,  whether  to  determine  their  power  of  distinguishing 
stimuli  or  their  power  of  learning  by  experience,  the  first 
requisite  is  to  give  the  animal  what  we  commonly  call  a 


Sensory  Discrimination  61 

motive.  That  is,  the  conditions  of  the  experiment  must  be 
so  arranged  that  some  already  present  tendency  to  act, 
whether  inborn  in  the  animal  or  acquired  by  previous  ex- 
perience, shall  be  appealed  to. 

This  is  increasingly  the  case,  the  higher  the  animal  worked 
with  stands  in  the  scale.  The  higher  animals  have  what 
might  be  called  a  large  reserve  fund  of  discriminations.  That 
is,  they  are  capable  of  making  many  more  selective  reactions 
to  stimuli  than  they  need  at  a  given  moment  actually  to  use. 
Hence  in  their  case  the  experimenter  must  make  a  careful  ad- 
justment of  conditions  to  bring  out  exactly  the  discrimination 
wanted.  He  must  either  make  the  performance  of  the  re- 
action pleasant  or  its  non-performance  unpleasant  to  the  ani- 
mal. A  monkey,  for  example,  confronted  by  a  set  of  glass 
tumblers  covered  each  with  a  differently  colored  paper,  may 
behave  toward  them  all  in  precisely  the  same  way;  yet  if 
food  be  put  regularly  in  the  blue  -tumbler,  whose  position  in 
the  row  is  varied,  it  becomes  worth  the  monkey's  while  to 
make  use  of  his  discriminative  powers,  and  he  may  show  by 
his  different  behavior  toward  the  blue  tumbler  that  it  pro- 
duces on  him  a  different  impression  from  the  others. 

With  simpler  animals  the  problem  is  less  difficult.     If 
an  animal  is  capable  only  of  a  half  dozen  different  ways  of 
responding  to  stimulation,  we  may  with  comparative  safety 
assume  that  it  has  less  opportunity  to  hold  them  in  reserve ; 
and  if  such  an  animal  invariably  reacts  in  the  same  way  to 
two  different  forms  of  stimulus,  or  if  the  variations  in  its  re- 
sponse are  not  correlated  with  differences  in  the  stimulation, 
it  becomes  probable  that  the  two  stimuli  produce  in  its  as- 
sumed consciousness  identical  sensation  qualities.     Thus  it  is. 
not  the  number  of  stimuli  to  which  an  animal  reacts  that! 
can  be  taken  as  evidence  of    the  qualitative  variety  of  itsf 
sensations,  but  the  number  of  stimuli  to  which  it  gives  dif- 


62  The  Animal  Mind 

ferent  reactions.  When  Jennings,  for  instance,  says  that 
Amoeba  "  reacts  to  all  classes  of  stimuli  to  which  higher 
animals  react"  (211,  p.  19),  we  cannot  conclude  that  it  pos- 
sesses all  classes  of  sensations  that  higher  animals  possess, 
for  its  reactions  to  these  different  stimuli  are  but  little  varied 
according  to  the  kind  of  stimulus.1 

§  1 6.   Evidence  from  Structure  and  Behavior  Combined 

As  a  matter  of  fact,  the  argument  from  structure  needs 
confirmatory  evidence  from  behavior.  For  clearly  the  mere 
presence  of  a  sense  organ  bearing  sufficient  likeness  to  our 
own  to  admit  of  conjecturing  its  function  would  be  of  no 
value  as  proof  unless  it  were  shown  that  the  sense  organ  actu- 
ally functioned.  In  order  to  do  this,  it  would  be  necessary  to 
show  that  the  animal  reacted  to  the  stimulus  conjectured  as 
appropriate  to  the  sense  organ,  and  that  removal  of  the  organ 
profoundly  modified  the  reaction.  Thus  we  shall  find  that 
many  experiments  to  test  sensory  discrimination  have  been 
made  by  the  method  of  extirpating  a  sense  organ  and  studying 
the  effect  on  behavior.  Thelnethod  has  many  disadvantages, 
the  chief  of  which  lies  in  the  fact  that  it  is  hard  to  say  which 
disturbances  in  behavior  are  due  actually  to  the  loss  of  the 
organ  and  which  to  the  more  widespread  effects  of  the  opera- 
tion. Yet  this  much  may  be  said  for  the  combination  of 
proof  from  structure  and  behavior  involved  in  the  Method 
of  Extirpation,  if  we  may  so  call  it :  where  an  animal  reacts 
to  a  certain  stimulus,  for  instance  light,  when  a  sense  organ  is 
intact,  and  fails  to  react  to  light,  though  otherwise  normal, 
when  the  organ  is  removed,  there  arises  a  possibility  that  light 

1  One  of  many  reasons  for  the  unsatisfactoriness  of  a  recent  article  by  A. 
Olzelt-Newin,  entitled  "  Beobachtungen  iiber  das  Leben  der  Protozoen" 
(299),  lies  in  the  author's  uncritical  acceptance  of  the  hypothesis  that  reac- 
tion to  a  special  kind  of  stimulus  means  a  special  kind  of  sensation. 


Sensory  Discrimination  63 

may  produce  in  the  animal's  consciousness  a  specific  sensation 
quality,  even  although  the  animal  ordinarily  reacts  to  light 
in  a  manner  indistinguishable  from  that  of  its  responses  to 
other  stimuli.  Though  light  and  mechanical  stimulation, 
for  example,  both  ordinarily  produce  a  negative  reaction, 
yet  if  light  brings  about  its  effect  only  through  the  medium 
of  a  specialized  structure  with  which  mechanical  stimuli 
are  not  concerned,  then  along  with  the  probable  un- 
pleasantness accompanying  the  negative  reaction  there  may 
go  a  quality  peculiar  to  the  functioning  of  that  special 
structure. 

Another  mode  of  combining  evidence  from  structure  with 
evidence  from  behavior  is  by  the  use  of  localized  stimuli.  If 
an  animal  gives  a  response,  which  in  itself  may  have  nothing 
to  mark  it  off  from  responses  to  other  stimuli,  when  a  special 
kind  of  stimulation  is  applied  to  certain  regions  of  the  body, 
and  only  then;  while  the  other  stimuli  produce  better  re- 
actions when  applied  elsewhere,  then  the  suggestion  is 
given  that  different  sense  organs  are  involved,  and  the  same 
possibility  arises  of  different  sensation  qualities. 

Two  other  forms  of  evidence  whereby  from  behavior  a 
differentiation  of  sensory  structures  can  be  argued,  and  from 
differentiation  of  sensory  structures  possible  differences  of 
sensation  quality,  may  be  mentioned.  The  first  of  these  con- 
sists in  showing  that  reactions  to  different  stimuli  may  be 
independently  fatigued.  The  natural  inference  is  that  a 
specific  nervous  apparatus  belongs  to  each  stimulus.  The 
second  lies  in  demonstrating  that  the  reactions  to  different 
stimuli  occur  with  different  degrees  of  rapidity.  If  there  is  a 
marked  difference  in  the  reaction  times  of  an  animal  to  different 
forms  of  stimulation,  each,  again,  may  be  supposed  to  affect 
its  own  nervous  pathway.  A  modification  of  this  method 
consists  in  noting  the  influence  of  a  stimulus  upon  the  time  of 


64  The  Animal  Mind 

reaction  to  another  nearly  simultaneous  stimulus.  If  such  an 
influence  can  be  shown,  it  is  evident  that  the  force  producing 
it  has  some  effect  on  the  nervous  system.  By  combining 
this  method  with  that  of  extirpating  a  sensory  structure, 
indications  may  be  obtained  that  the  nervous  effect  of  the 
auxiliary  stimulus  is  dependent  on  a  definite  receptive 
apparatus,  and  hence  is  probably  accompanied  by  a  special 
sensation.  This  method  was  used  by  Yerkes  to  demonstrate 
hearing  in  frogs  (456,  462,  464). 

One  further  consideration  offers  itself  to  the  student  of 
animal  responses  to  stimulation.  It  has  been  the  special  en- 
deavor of  Jennings  to  point  out  the  fact  that  these  responses, 
instead  of  being  wholly  accounted  for  by  the  characteristics 
of  the  stimulus,  are  determined  in  part  by  the  internal, 
physiological  condition  of  the  animal  (211).  We  shall 
therefore  note  often  in  the  course  of  the  following  pages  cases 
where  difference  of  reaction  is  due  to  internal  rather  than  to 
external  causes. 

§  17.   Evidence  for  Discrimination  of  Certain  "Lower" 
Sensation  Classes 

Bearing  all  these  points  in  mind,  let  us  proceed  to  survey 
the  evidence  for  variety  in  the  sensations  of  animals.  In 
the  lowest  forms,  such  evidence  must  be  derived  entirely  from 
behavior.  That  from  the  presence  of  a  sense  organ  is  almost 
wholly  lacking.  And  although  various  stimuli,  as  we  have 
seen,  produce  reactions  in  Amoeba,  yet  there  is  only  one  case 
where  these  reactions  are  strikingly  different  according  to  the 
quality  of  the  stimulus  applied.  This  instance  consists  in  the 
distinction  between  food-taking  reactions,  given  to  edible  sub- 
stances, and  the  responses  to  mechanical  stimulation.  The 
sense  of  touch,  undoubtedly,  must  play  a  part  in  the  mental 
life  of  the  lowest  animals  that  have  consciousness  at  all. 


Sensory  Discrimination  65 

But  the  earliest  distinction  between  a  touch  quality  and  a 
quality  that  is  other  than  touch  seems  to  occur  when  food 
sensation  and  contact  sensation  are  differentiated.  It  is 
possible  that  warmth  and  cold  also  appear  as  distinct  sensation 
qualities  in  the  experience  of  low  forms  of  animals,  but  we 
have  little  real  evidence  of  the  fact.  No  organs  of  tempera- 
ture sensation  are  definitely  known  even  in  human  beings. 
And  the  responses  of  low  animals  to  thermal  stimulation  are 
not  specialized.  They  consist  usually  of  negative  reactions, 
given  when  the  animal  is  subjected  to  a  temperature  either 
above  or  below,  but  especially  above,  the  " optimum";  and 
these  reactions  are  not  different  from  the  ordinary  negative 
type,  suggesting  unpleasantness  rather  than  a  specific  sensa- 
tion quality.  In  some  cases  the  sensibility  to  thermal  stimu- 
lation has  been  found  to  be  differently  distributed  from  that 
to  other  classes  of  stimuli.  But  in  any  case,  sensations  of 
warmth  and  cold  are  probably  in  no  member  of  the  animal 
kingdom  differentiated  into  any  greater  number  of  qualita- 
tively distinct  sensations. 

The  sense  of  touch,  also,  shows  but  little  internal  differen- 
tiation. Its  importance,  so  far  as  we  can  judge,  is  rather  on 
the  spatial  than  on  the  qualitative  side.  The  sense  quality 
of  pain  we  naturally  think  of  as  the  accompaniment  of  the 
negative  reaction  in  its  more  violent  forms,  given  to  a  stimulus 
that  is  injuring  the  organism.  Organic  and  kinaesthetic 
sensations  are  hard  to  trace  in  the  lower  animals ;  for  animals 
whose  structure  differs  widely  from  our  own,  the  qualities 
of  these  two  classes  must  remain  beyond  the  power  of  our 
imagination.  That  differences  in  physiological  condition 
such  as  are  produced  by  hunger,  satiety,  or  fatigue  involve 
differences  of  accompanying  organic  sensation  in  the  con- 
sciousness of  the  animal  manifesting  them  is  possible. 
Kinsesthetic  sensations,  as  we  shall  see,  are  apparently  con- 

F 


66  The  Animal  Mind 

cerned  in  the  processes  whereby  many  animals  have  learned 
to  traverse  a  labyrinth  path. 

The  three  classes  of  sensation  whose  existence  in  the  ani- 
mal mind  can  be  most  satisfactorily  traced  are  the  chemical 
sense,  under  which  smell  and  taste  belong,  the  sense  of  hear- 
ing, and  the  sense  of  sight.  To  the  study  of  these  the  follow- 
ing chapters  will  be  devoted.  Since  the  manifestations  of  the 
chemical  sense  in  the  lowest  forms  of  animals  consist  chiefly 
in  a  differentiation  of  response  to  food  and  to  mechanical 
stimulation,  the  contact  sense  or  sense  of  touch  will,  in  dis- 
cussing these  forms,  be  considered  along  with  the  chemical 
sense. 


CHAPTER  V 

SENSORY  DISCRIMINATION:   THE  CHEMICAL  SENSE 

§  1 8.     The  Chemical  Sense  in  Ccdenterates 

WE  have  already  discussed  the  responses  to  mechanical, 
chemical,  and  food  stimulation  in  those  members  of  the  Pro- 
tozoa whose  behavior  has  been  most  carefully  studied,  and 
may  begin  the  present  chapter  with  an  account  of  the  corre- 
sponding reactions  in  the  lowest  of  the  Metazoa,  or  many- 
celled  animals,  the  coelenterates.  Although  externally  the 
forms  of  different  families  of  coelenterates  differ  widely,  yet 
the  general  plan  of  structure  is  the  same  in  all :  the  body  of 
the  typical  ccelenterate  is  a  hollow  sac,  whose  walls  consist 
of  two  layers  of  cells,  food  being  taken  into  a  mouth  at  one 
end  of  the  sac,  and  the  arrangement  of  cells  being  on  the  plan 
of  circular  symmetry.  In  the  phylum  of  the  coelenterates  are 
included  sea-anemones,  jellyfish,  the  little  green  or  yellow 
Hydra,  sponges,  corals,  and  ctenophores. 

Hydra  (Fig.  6),  one  of  the  simplest  coelenterates,  shows 
a   food   reaction  distinct   from  the  contact  reaction.     Me- 
chanical stimulation  is  followed  by  withdrawal  of  the  ten- 
tacles, and  by  contraction  of  the  stem.     This  behavior  may  , 
be  called  a  negative  or  avoiding  reaction,  and  no  positive^ 
reaction  to  a  mechanical  stimulus  has  been  observed.     The 
food-taking  reaction,  on  the  other  hand,  consists  in  the  seiz- 
ing of  the  food  by  the  tentacles.     It  seems  to  be  given  in 
response   to  a   combination   of  chemical  wjth    mechanical 
stimulation,  such  as  is  offered  by  contact  with  a  solicTedible 

67 


68 


The  Animal  Mind 


object  (418).  Shall  we  say  that  Hydra  possesses,  then,  a 
food  sensation  and  a  contact  sensation  that  are  distinguish- 
able in  its  consciousness,  provided  such  consciousness  exists  ? 
It  may  be  that  the  contrast  between  the  two  is  more  nearly 
analogous  to  that  between  pleasantness  and  unpleasantness 
in  our  own  experience,  for  the  food-taking  reaction  in  Hydra 

is  the  only  form  of  the 
positive  reaction,  and  the 
response  to  mere  contact 
is  distinctly  negative  in 
character.  The  influence 
of  physiological  condition 
in  Hydra's  reactions  is 
shown  by  the  fact  that 
although  ordinarily  the 
food  response  is  brought 
about  only  by  contact 
with  food,  if  the  animal 
is  very  hungry  any 
chemical  stimulation, 
even  quinine,  will  pro- 
duce it  (418).  This 
blunting  of  discrimina- 
tion has,  of  course,  the 
adaptive  aspect  that  the 
starved  animal  can  afford  to  lose  no  chances,  and  suggests  the 
analogy  from  our  own  experience  of  the  loss  of  intellectual 
discrimination  in  moments  of  intense  emotion.  For  the 
emotion  too  represents  a  situation  where  the  organism  can- 
not afford  to  lose  chances  by  hesitating  in  reaction  long 
enough  for  nice  discrimination. 

In  Tubularia  crocea,  a  ccelenterate  belonging  to  the  family 
of  hydroids  which  form  colonies  of  many  individuals  on  a 


FIG.  6.  —  Hydra,    mth,  mouth;  *,  tentacle. 
After  Parker. 


Sensory  Discrimination:  the  Chemical  Sense    69 

common  stem,  food  and  contact  stimuli  do  not  produce  dif- 
ferent reactions,  but  have  different  degrees  of  efficiency  in  V 
bringing  about  response.  When  a  grain  of  sand  was  placed  in 
contact  with  the  tentacles  on  one  side  and  a  bit  of  meat  in  a 
corresponding  position  on  the  other  side,  the  reaction  was 
almost  invariably  in  the  direction  of  the  meat.  Filtered  meat 
juice  allowed  to  flow  upon  the  distal  tentacles  produced  a  reac- 
tion 82  per  cent  of  the  time,  while  carmine  water  was  effective 
only  15  per  cent  of  the  time.  Further,  if  the  distal  tentacles 
were  touched  several  times  with  a  needle,  they  remained 
closed ;  but  if  the  second  stimulus  used  was  a  piece  of  meat, 
the  tentacles  opened  out  and  waved  about  (319).  Whether 
in  such  a  case  as  this  the  possible  conscious  accompaniments 
of  the  responses  are  to  be  regarded  as  qualitatively  different /f 
sensations,  or  only  as  different  degrees  of  intensity  of  the  same 
sensation,  it  is  difficult  to  say.  Another  hydroid,  Corymorpha 
palma,  gives  no  response  whatever  to  meat  juice;  only  irri- 
tating chemicals  produce  reactions,  whose  character  appears 
to  be  tactile  (402). 

In  the  sea- anemones  or  actinians  we  find  behavior  in  re- 
sponse to  food  stimulation  as  distinguished  from  contact 
stimulation  varying  in  different  representatives  of  the  group. 
Generally  speaking,  the  food  reaction  seems  to  be  more  marked 
than  the  contact  reaction.  W.  H.  Pollock  a  number  of  years 
ago  reported  his  observation  that  certain  unnamed  sea- anem- 
ones opened  out  if  food  were  suspended  near  them  in  the 
water,  and  referred  the  phenomenon  to  "a  sense  of  smell" 
(343).  Adamsia  rondeleti  winds  its  tentacles  around  bits 
of  sardine  meat  and  passes  them  from  tentacle  to  tentacle 
toward  the  mouth.  When  balls  of  filter  paper  softened  with 
sea  water  are  substituted,  the  feeding  reaction  is  wholly  lack- 
ing. Either  the  tentacles  fail  to  react  at  all,  or  the  ball  is 
"felt  of "  slowly  with  no  attempt  to  seize  it,  or  it  is  momentarily 


7<D  The  Animal  Mind 

seized  and  then  dropped.  If  the  paper  ball  be  soaked  in  fish 
juice,  on  the  other  hand,  it  is  seized  as  eagerly  as  the  fish 
meat.  A  negative  reaction,  consisting  in  the  withdrawal  of 
the  tentacles  affected,  may  be  produced  by  applying  a  bit 
of  paper  soaked  in  quinine  solution  or  by  the  discharge  of  qui- 
nine solution  from  a  pipette  near  the  tentacles  (237,  288).  A 
peculiar  form  of  negative  reaction  has  been  observed  in  Adam- 
sia,  and  more  strikingly  in  Cerianthus,  when  a  paper  ball  soaked 
in  fish  juice  has  been  passed  from  tentacle  to  tentacle  till  it 
has  nearly  reached  the  mouth.  The  process  is  suddenly  re- 
versed, and  the  ball  is  passed  back  from  one  tentacle  to  another 
till  it  reaches  the  outside  edge  and  is  dropped  off.  Nagel, 
the  observer,  thinks  the  stimulus  for  this  change  of  reaction 
is  the  gradual  wearing  off  of  the  "sapid  parts"  of  the  ball 
during  its  passage  toward  the  mouth  —  it  might  be  the 
squeezing  out  of  the  meat  juice  —  and  calls  special  atten- 
tion to  the  fact  that  the  reaction  whereby  the  paper  is  got  rid 
of  is  wholly  different  from  the  ordinary  reaction  of  a  tentacle 
to  mechanical  stimulation,  which,  as  we  have  seen,  does  not 
involve  seizing  the  object  at  all.  A  tentacle  touched  by  a  bit 
of  moistened  filter  paper  ordinarily  responds,  if  at  all,  by  a 
mere  contraction  without  the  winding  seizure  of  the  object. 
Touched  by  the  same  object  "handed  on"  to  it  by  a  tentacle 
nearer  the  mouth  than  itself,  it  seizes  the  paper  and  passes  it 
on  to  the  tentacle  beyond  it.  The  cause  of  this  difference  in 
behavior  seems  to  lie  in  the  processes  that  have  been  taking 
place  just  previously.  Nagel  does  not  hesitate  to  say  that  a 
psychic  process  must  be  involved,  but  its  details  are  not  easy 
to  construct  (291). 

Another  sea-anemone,  Aiptasia,  has  but  one  ring  of  ten- 
tacles, and  like  Tubularia  crocea,  instead  of  showing  different 
responses  to  contact  stimulation  alone  and  to  contact  plus  food 
stimulation,  it  merely  reacts  with  greater  emphasis  to  the 


Sensory  Discrimination :  the  Chemical  Sense     7 * 


latter.  In  both  cases  the  tentacles  wind  around  the  object, 
contract,  and  direct  themselves  toward  the  mouth  (291). 
Again  the  question  arises  whether  the  possible  accompanying 
sensations  differ  in  quality  or  only  in  intensity.  One  species 
of  Aiptasia,  A.  annulata,  however,  does  react  differently 
to  filter  paper  soaked  in  crab  juice  and  to  plain  filter  paper 
(207),  showing  that  even  within  a  genus  the  capacity  for  stim- 
ulus discrimination  may  differ.  In  like  manner  one  sea- 
anemone,  Actinia,  will  take  filter  paper  soaked  in  acetic  acid, 
while  another,  Tealia,  rejects  it  (127). 

Metridium,  a  common  sea-anem- 
one of  our  coasts,  has  its  tentacles 
covered  with  cilia  which  have  a  con- 
tinual waving  motion  toward  the  tip 
of  the  tentacle  (Fig.  7).  If  particles 
of  an  inedible  substance  are  dropped 
on  a  tentacle,  no  definite  reaction 
occurs,  but  the  particles  are  carried 
by  the  ordinary  motion  of  the  cilia 
out  to  the  tentacle  tip,  where  they  drop  off.  When  a  bit  of 
crab  meat,  or  some  meat  juice,  is  dropped  on  a  tentacle,  the 
latter  contracts  and  curls  over  with  the  tip  directed  toward 
the  mouth.  The  ciliary  movement  continuing  in  its  usual 
direction  now  of  course  carries  the  food  toward  the  mouth. 
Applying  food  to  the  lips  on  either  side  of  the  mouth  causes 
a  different  response.  The  cilia  on  these  lips  ordinarily  wave 
outwards;  when  food  is  brought  in  contact  with  them  their 
motion  is  reversed,  and  the  food  is  thus  passed  into  the 
mouth.  In  Metridium,  then,  there  is  no  specific  rejecting 
reaction  for  inedible  substances  (303). 

Various  instances  of  the  effect  of  physiological  condition 
upon  response  to  food  stimulation  in  sea-anemones  have  been 
noted.  Adamsia  loses  the  power  to  discriminate  between  edi- 


FIG.  7.  —  Metridium.    After 
Parker. 


72  The  Animal  Mind 

ble  and  inedible  substances  when  very  hungry  (291).  Sagar- 
tia  davisi  will  also  swallow  inedible  substances  if  hungry 
enough  (403).  Stoiachactis  helianthus  will  give  either  a 
positive  or  a  negative  reaction  to  food  according  to  its  condi- 
tion of  hunger  or  satiety  (207).  The  reaction  of  Metridium 
to  food  may  vary  decidedly  with  the  degree  of  hunger  (3) ; 
although  it  will  continue  taking  food  as  long  as  the  process  is 
mechanically  possible  (211).  Fatigue  has  also  been  shown 
to  affect  the  food  responses  of  Metridium  and  other  sea- 
anemones  ;  specimens  that  have  been  fed  meat  and  filter  paper 
alternately  will  after  a  time  refuse  to  take  filter  paper  (207, 
291,  303).  This  behavior  was  thought  by  Nagel  to  indicate 
that  the  animal  had  discovered  the  deception  practiced  upon 
it ;  but  apparently  the  real  cause  is  fatigue ;  showing  itself  first 
T»  with  reference  to  the  weaker  of  the  two  stimuli  (3). 

As  regards  the  localization  of  the  sensitive  elements,  au- 
thorities, and  probably  species,  differ.  Nagel  finds  the  ten- 
tacles most  sensitive  (291) ;  Loeb  observed  that  the  stump  of 
the  animal  has  discriminative  reactions  (237),  while  Fleure 
and  Walton  state  that  in  the  species  tested  by  them  the  mouth- 
region  is  most  responsive  to  chemical  stimulation  (127). 

A  certain  amount  of  discrimination  between  mechanical 
stimuli  is  asserted  of  these  animals  by  Romanes.  "I  have 
observed,"  he  says,  "  that  if  a  sea- anemone  is  placed  in  an 
aquarium  tank  and  allowed  to  fasten  upon  one  side  of  the 
tank  near  the  surface  of  the  water,  and  if  a  jet  of  sea  water 
is  made  to  play  continuously  and  forcibly  upon  the  anemone 
from  above,  the  result  of  course  is  that  the  animal  becomes 
surrounded  with  a  turmoil  of  water  and  air  bubbles.  Yet 
after  a  short  time  it  becomes  so  accustomed  to  this  turmoil 
that  it  will  expand  its  tentacles  in  search  of  food,  just  as  it  does 
when  placed  in  calm  water.  If  now  one  of  the  expanded  ten- 
tacles is  gently  touched  with  a  solid  body,  all  the  others  close 


Sensory  Discrimination:  the  Chemical  Sense     73 

around  that  body  in  just  the  same  way  as  they  would  were  they 
expanded  in  calm  water "  (366,  p.  48),  although  the  solid 
stimulus  is  decidedly  less  intense  than  that  offered  by  the 
bubbles.  Similarly,  Fleure  and  Walton  find  that  certain 
species  show  little  reaction  to  accidental  contact  with  a  pebble 
that  is  moved,  but  react  quickly  to  a  finger  (127). 

The  body  of  a  typical  medusa  or  jellyfish  consists  of  a  bell- 
shaped  "  umbrella  "  from  the  edge  of  which  tentacles  depend. 
Hanging  from  the  middle  like  the  clapper  of  the  bell  or  the 
handle  of  the  umbrella  is  the  manubrium,  at  the  end  of  which 
is  the  mouth.  In  the  medusa  Carmarina  hastata  no  differen- 
tiation in  reaction  to  contact  and  food  stimulation  appears, 
merely  a  readier  response  of  the  tentacles  to  the  latter ;  but 
we  do  find  whatever  evidence  for  the  existence  of  a  specific 
sensation  quality  is  furnished  by  localized  sensitiveness,  for 
the  skin  of  the  under  side  of  the  umbrella,  and  of  the  manu- 
brium, is  very  sensitive  to  mechanical  stimulation,  and  wholly 
insensitive  to  chemical  stimulation,  while  the  tentacles,  as 
has  just  been  stated,  react,  by  shortening  and  twisting  them- 
selves about  the  object,  more  readily  to  chemical  than  to 
mechanical  stimulation.  A  mechanical  stimulus  applied  to 
any  part  of  the  under  edge  of  the  umbrella  produces  after 
from  one  to  three  seconds  a  movement  of  the  manubrium  tip 
toward  the  point  stimulated  (289,  291). 

The  little  medusa  Gonionemus  murbachii  (Fig.  8)  shows,  on 
the  other  hand,  two  well-defined  different  responses  to  special 
stimulation :  motor  reactions  and  food-taking  reactions.  The 
motor  or  swimming  reactions  are  given  in  response  to  me- 
chanical stimulation  and  to  the  presence  of  food  near  the 
animal  in  the  water;  but  the  food-taking  reaction  occurs 
only  in  response  to  food  (solution  of  fish  meat) ;  very  rarely 
a  weak  inorganic  chemical  stimulus  will  produce  the  begin- 
ning of  the  response.  An  important  exception  to  the  usual 


74 


The  Animal  Mind 


inefficacy  of  mechanical  stimuli  in  bringing  about  the  feeding 
reaction  occurs  when  a  moving  mechanical  stimulus  is  used  ; 
this  very  quickly  produces  the  early  stages  of  the  food-taking 
response.  Special  reactions  to  stimuli  in  motion  are  wide- 
spread throughout  the  animal  kingdom;  their  significance 
will  be  discussed  in  the  chapter  on  Space  Perception.  The 
food-taking  response  in  Gonionemus  shows  a  marked  coor- 
dination of  movements;  if  the  food  touches  one  or  more 
tentacles,  these  contract  and  twist  about  it  ;  they  then  bend 
toward  the  manubrium,  and  the  margin  of  the  bell  also 

bends  in  ;  the  manubrium 
swings  over  toward  the 
bell  and  envelops  the 
food  with  its  lips  (451). 
Another  ccelenterate 
whose  reactions  to  chemi- 
cal stimulation  have  been 
observed  is  the  cteno- 

FIG.  8.  -Gonionemus.    After  Hargitt.  ph()re 


body  is  an  elongated  oval,  with  longitudinal  ciliated  ridges, 
having  the  mouth  slit  at  the  end  which  is  normally  uppermost 
when  the  animal  is  at  the  surface  of  the  water,  and  at  the 
opposite  end  an  otolith  or  statolith  organ  lying  between  two 
flattened  "  polar  plates."  The  significance  of  this  organ  will 
be  considered  later.  The  aboral  region  is  far  more  sensitive 
than  any  other  to  mechanical  stimulation  ;  the  slightest  touch 
on  one  of  the  polar  plates  causes  the  animal  to  shorten  itself 
and  fold  in  the  plates.  The  aboral  end,  being  the  hind  end  of 
the  creature,  is  not  usually  brought  into  contact  with  objects. 
Nagel,  who  studied  the  animal,  suggests  that  this  region,  being 
sensitive  to  changes  in  pressure,  may  enable  the  animal  to 
right  itself  when  it  rises  to  the  surface  with  the  aboral  end 
up,  as  the  change  from  water  to  air  pressure  could  not  fail 


Sensory  Discrimination :  the  Chemical  Sense     75 

to  stimulate  the  polar  plates.  Nagel  apparently  made  no 
experiments  on  the  behavior  of  Beroe  with  reference  to  food 
stimuli;  for  chemical  stimulation  he  used  picric  acid,  dilute 
hydrochloric  acid,  quinine,  strychnine,  saccharine,  coumarin, 
vanillin,  and  naphthalin.  To  all  these  unwonted  stimuli  the 
animal  responded  by  some  form  of  negative  reaction,  indicat- 
ing possible  unpleasant  feeling.  The  edges  of  the  mouth, 
where  the  nerves  end  in  bulblike  structures,  reacted  to 
quinine,  vanillin,  and  coumarin  by  stretching  the  mouth  into 
a  circular  form  instead  of  its  usual  slitlike  shape ;  suggesting 
an  effort  to  get  rid  of  the  stimulus.  Precisely  similar  re- 
actions were  produced  by  stimulation  with  lukewarm  water. 
Nagel  concludes  that  the  organs  for  chemical  and  ther-y^ 
mal  stimulation  are  identical;  whether  the  sensation  quali-' 
ties  are  different  is,  he  thinks,  an  open  question.  There  is 
at  least  no  evidence  that  they  are  different  (289,  291). 

§  19.  The  Chemical  Sense  in  Flatworms 
Next  to  the  coelenterates  zoologists  place  the  phylum  of  the 
Platyhelminthes  or  flatworms,  which  possess  a  bilaterally 
instead  of  a  radially  symmetrical  structure.  Many  repre- 
sentatives of  the  group  are  parasitic,  and  so  far  as  the  writer 
is  aware,  no  extended  study  of  the  reactions  of  these  forms  to 
stimulation  has  been  made.  Most  of  our  knowledge  in  re- 
gard to  the  sensory  life  of  the  flatworms  is  confined  to  the  class 
Turbellaria,  including  the  common  freshwater  and  marine 
planarians.  These  are  small  slow-moving  creatures  which 
crawl  about  on  solid  objects  under  water  or  on  films  covering 
the  surface.  The  mouth  is  situated  on  the  ventral  side  of 
the  body,  sometimes  quite  far  removed  from  the  head  end 
(Fig.  9).  One  chief  interest  of  planarians  to  physiologists 
has  lain  in  their  remarkable  power  to  regenerate  parts  lost 
by  mutilation. 


76 


The  Animal  Mind 


Planaria  maculata,  a   common  freshwater  planarian,  re- 
v,  sponds  to  stimulation  by  two  forms  of  negative  reaction,  a  posi- 
'•  tive  reaction,  and  a  feeding  reaction.     The  negative  and  posi- 
tive responses  are  given  either  to  mechanical  or  to  chemical 
stimuli,  the  former  being  produced  by  strong, 
the  latter  by  weak  stimulation.     Hence  they 
.do  not  suggest  correlation  with  qualitatively 
7  different  sensation  contents,  but  rather  with 
unpleasantness   and   pleasantness.     The   two 
forms  of  negative  reaction  correspond  to  dif- 
ferences in  the  location  of  the  stimulus.     If  the 
head  end  of  the  body  is  stimulated  strongly  on 
one  side,  the  head  is  turned  awray  from  that 
side.     If  the  posterior  part  of   the  body  is 
strongly  stimulated,  the  animal  makes  power- 
ful forward  crawling  movements.     The  signifi- 
cance of  local  differences  in  stimulation  for 
response  and  for  possible  consciousness,  again, 
will  more   properly  be  discussed  in  a  later 
chapter.     As  has  just  been  said,  both  weak 
chemical   and   weak   mechanical   stimulation 
cause  Planaria  maculata  to  give  a   positive 
reaction  by  turning  its  head  in  the  direction  of 
the  stimulus,  which  need  not  be  in  actual  con- 
tact with  the  body  (316).     A  planarian  will 
follow  an  object  such  as  the  point  of  a  pin 
moved  in  front  of  it,  and  one  planarian  will 
follow  the  trail  of  another  that  happens  to  come  within  the 
proper  distance.     Similarly,  the  neighborhood  of  food  will 
cause  the  animal  to  turn  toward  it.     Bardeen  has  suggested 
that  the  so-called  "auricular  appendages,"  two  small  movable 
prominences  on  the  animal's  back  near  the  head  end,  which 
are  specially  sensitive   to  touch,  may  be  "  delicate  organs 


FIG.  9.— Plana- 
rian, dorsal 
view.  After 
Woodworth. 


Sensory  Discrimination :  the  Chemical  Sense     77 

capable  of  stimulation  by  slight  currents  in  the  water  set  up  by 
the  minute  organisms  that  prey"  upon  the  animal's  food;  so 
that  the  positive  reaction  when  given  to  food  may  be  really  a 
response  to  mechanical  stimulation  (10).  As  Pearl,  however, 
found  that  chemicals,  diffused  in  the  water,  would  produce 
positive  responses  (316),  it  is  probable  that  Planaria  maculata 
is  directly  sensitive  to  chemical  stimulation,  though  it  re- 
sponds thereto  in  the  same  way  as  to  mechanical  stimulation. 
A  land  planarian,  Geodesimus  bilineatus,  is  reported  by 
Lehnert  to  perceive  food  at  distances  from  four  to  five  times  the 
length  of  its  body,  and  he  does  not  describe  the  positive  reaction 
as  given  in  response  to  any  other  than  food  stimulation  (231). 

The  food-taking  reaction  in  Planaria  maculata  is  made 
under  the  influence  of  combined  mechanical  and  chemical 
stimuli,  in  contact  with  the  pharynx  or  the  ventral  side  of  the 
animal.  When  an  object  which  has  occasioned  the  positive 
reaction  is  reached,  the  head  folds  over  it  and  grips  it,  contract- 
ing so  as  to  squeeze  it.  The  substance  being  thus  brought  into 
contact  with  the  pharynx,  swallowing  movements  are  pro- 
duced if  the  proper  stimulus  is  given.  Bardeen  was  inclined 
to  think  that  contact  with  a  soft  substance  constituted  the 
proper  stimulus,  as  he  found  that  hard  particles  placed  on  the 
pharynx  were  not  swallowed  (9).  Pearl,  however,  believes 
that  mechanical  and  chemical  stimulation  must  combine. 
The  former  alone  does  not  suffice,  for  swallowing  movements 
are  not  evoked  when  one  planarian  crawls  over  another;  the 
latter  alone  is  insufficient,  for  placing  the  animal  in  a  sugar 
solution  has  no  effect.  If  chemical  and  mechanical  stimula- 
tion are  united,  the  reaction  is  given  whether  the  chemical  is 
edible  or  not ;  Pearl  found  it  occurring  in  response  to  sodium 
carbonate  (316). 

Evidence  of  the  influence  of  physiological  condition  upon 
the  reactions  of  planarians  is  furnished  by  the  fact  that  the 


78  The  Animal  Mind 

resting  planarian  shows  a  decidedly  lowered  susceptibility  to 
stimulation.  Bardeen  found  that  if  the  animal  was  not 
already  in  motion,  it  gave  no  positive  response  to  food  in 
its  neighborhood  (10). 

§  20.     The  Chemical  Sense  in  Annelids 

In  our  own  experience,  as  has  been  said,  the  "food  sense" 
is  represented  by  the  two  senses  taste  and  smell,  the  stimulus 
for  the  one  being  fluid,  and  that  for  the  other  gaseous,  so  that 
the  latter  enables  us  to  perceive  objects  at  a  distance.  For 
water-dwelling  animals,  such  as  most  of  those  whose  behavior 
,  we  have  been  describing,  the  distinction  evidently  cannot 
'  well  be  drawn.  If  such  an  animal  perceives  food  at  a  dis- 
tance, the  stimulus  is  necessarily  diffused  through  the  water, 
and  Lloyd  Morgan  has  proposed  the  term  " telaesthetic  taste" 
for  the  sense  which  makes  such  perception  possible  (279, 
p.  256).  The  term  indicates  that  this  sense  corresponds  to 
taste  in  an  air-dwelling  animal  because  the  stimulus  is  fluid, 
but  differs  in  that  it  allows  perception  of  a  distant  object,  as 
taste  in  the  ordinary  sense  does  not.  In  the  most  familiar 
representative  of  the  Annelida  or  segmented  worms,  the  com- 
mon earthworm,  as  in  the  land  planarian,  a  distinction  analo- 
gous to  that  between  smell  and  taste  in  our  own  sensory 
experience  may  be  made ;  in  the  leeches  and  marine  annelids 
it  cannot. 

Gentle  and  continuous  mechanical  stimulation  produces  in 
the  earthworm  "positive  thigmotaxis " ;  that  is,  the  animals 
have  a  tendency  to  crawl  and  lie  along  the  surface  of  solids 
(387).  That  there  is  some  discrimination  of  edible  from 
inedible  substances  when  in  contact  with  the  body  Darwin 
thought  probable  from  the  apparent  preference  of  the  worm 
for  certain  kinds  of  food  (91).  In  the  earthworm  Allolobo- 
phora  fatida  we  find  a  differentiated  response  to  contact 


Sensory  Discrimination:  the  Chemical  Sense     79 

and  chemical  stimulation.  These  worms  live  in  barnyard 
manure.  When  placed  on  scraps  of  shredded  filter  paper 
moistened  with  water  they  refuse  to  burrow;  when  the  filter 
paper  is  wet  with  a  decoction  of  the  manure  they  burrow  as 
soon  as  they  come  into  contact  with  it.  The  adequate  stimu- 
lus for  burrowing  is  thus  a  combined  mechanical  and  chemical 
one;  the  chemical  stimulus  alone  is  insufficient,  for  filter 
paper  thus  prepared  has  no  effect  on  the  worms  unless  they 
are  actually  in  contact  with  it  (387).  Using  the  human  terms, 
the  case  is  one  of  taste  rather  than  smell.  Nagel  suggests 
that  the  earthworm's  chief  use  for  a  chemical  sense  is  to  help 
it  find  the  moisture  which  is  necessary  to  its  life  (292) ;  but 
curiously  enough  Allolobophora  fatida  seems  to  have  no  power 
of  doing  this  from  a  distance.  Smith  found  that  a  worm 
would  crawl  around  a  wet  spot  on  paper  until  its  skin  dried, 
without  crawling  into  it.  If  by  accident  it  happened  to  touch 
the  moist  place,  it  would  enter  and  remain  there  (387).  There 
seems  no  satisfactory  evidence  that  worms  respond  to  chemicaL 
stimulation  from  a  distance  by  positive  reactions,  although/ 
Darwin  believed  that  they  found  buried  food  by  "the  sense 
of  smell"  (91).  Chemical  stimuli  not  in  contact  with  the 
body  do  produce  negative  reactions  (292),  but  these  reactions  X 
do  not  differ  from  the  responses  to  strong  mechanical  stimu- 
lation. They  are  of  various  forms  —  turning  aside,  with- 
drawing into  the  burrow  if  the  tail  is  already  inserted,  squirm- 
ing, and  so  on,  the  differences  being  correlated  with  differences 
in  the  intensity  and  location  of  the  stimulus  and  in  the  excita- 
bility (physiological  condition)  of  the  animal.  But  nothing 
in  the  character  of  the  response  suggests  that  negative  reaction 
to  a  chemical  stimulus  has  a  different  conscious  accompani- 
ment from  that  of  negative  response  to  a  mechanical  stimulus. 
The  most  natural  interpretation  of  them  all  on  the  psychic 
side  is  that  of  unpleasantness,  increasing  in  intensity  as  the  X 


8o  The  Animal  Mind 

reaction  takes  a  more  violent  form.1  The  time  occupied  in 
reacting  has,  however,  recently  been  made  a  basis  for  dif- 
ferentiating the  response  to  different  chemicals.  It  was  found 
that  if  the  worms  were  suspended  by  threads,  and  their  an- 
terior ends  dipped  into  solutions  of  sodium,  ammonium, 
lithium,  and  potassium  chlorides,  the  animals  reacted  to  these 
substances  with  diminishing  promptness  in  the  order  just 
given.  The  differences  in  reaction  time  were  marked.  Now 
all  four  of  these  substances  produce  in  man  nearly  the  same 
taste  quality,  salt,  for  which  the  common  constituent  chlorine 
is  therefore  held  responsible.  The  sodium,  lithium,  ammo- 
nium, and  potassium  ions  have  apparently  but  little  effect  on 
the  human  taste  organs.  Since  the  earthworm  reacts  with 
decided  time  differences  to  the  four,  it  may  be  that  its  taste 
organs  are  specifically  affected  by  each,  and  that  different  taste 
Y  qualities  may  be  occasioned  in  its  consciousness,  supposing  it 
/  to  be  conscious  (314).  In  other  members  of  the  annelid  family, 
such  as  the  leeches  and  marine  worms,  we  know  little  of  the 
differentiation  between  food  and  contact  senses.  That  some  of 
them  respond  to  odorous  substances  is  stated  by  Nagel  (292). 

§  21.     The  Chemical  Sense  in  Mollusks 

In  the  case  of  the  Mollusca,  also,  there  is  little  satisfactory 

r  evidence  on  the  subject  of  the  chemical  sense.    The  Acephala, 

to  which  the  clam,  oyster,  and  scallop  belong,  do  not  take 

1  W.  W.  Norman  argued  that  the  squirming  reactions  of  worms,  and 
the  corresponding  reactions  of  other  animals  to  injurious  stimulation,  can- 
not be  taken  as  evidence  of  an  accompaniment  of  disagreeable  conscious- 
ness, because  of  the  fact  that  when  the  worm,  for  instance,  is  cut  in  two, 
the  squirming  movements  are  confined  to  the  posterior  piece,  while  the  head 
end  crawls  away  undisturbed.  The  head  end,  he  urges,  containing  the 
cerebral  ganglia,  ought  to  be  the  part  capable  of  suffering,  but  it  gives  no 
reaction  (295).  We  cannot,  however,  conclude  from  the  absence  of  a  reac- 
tion under  abnormal  conditions  that  its  possible  conscious  accompaniment 
in  the  normal  state  is  also  done  away  with. 


Sensory  Discrimination:  the  Chemical  Sense    81 

food  by  active  movements ;  hence,  of  course,  they  can  have  no 
specific  feeding  reactions.  Chemical  sensibility,  distributed 
over  the  surface  of  the  body,  has  been  observed  in  lamelli- 
branchs,  a  branch  of  the  Acephala  (292).  Gasteropods,  in- 
cluding snails  and  slugs,  have,  owing  to  their  active  food 
taking,  more  use  for  a  chemical  sense;  in  marine  snails  it 
seems  rather  definitely  localized  in  the  feelers  (292).  Yung 
found  in  the  snail  Helix  pomatia  that  smell  was  most  acute 
at  the  end  of  the  feelers,  but  that  the  animal  even  when 
deprived  of  its  feelers  could  distinguish  perfume.  Taste 
he  found  best  developed  near  the  lips,  and  touch  sensibility 
distributed  over  the  body,  but  especially  toward  the  end  of 
the  feelers  (472,  474). 

§  22.     The  Chemical  Sense  in  Echinoderms 

In  the  phylum  of  the  echinoderms,  under  which  are  classed 
starfish  and  sea-urchins,  the  " circular  symmetry"  of  body 
structure  characteristic  of  the  ccelenterates  reappears.  Star- 
fish were  found  by  Romanes  many  years  ago  to  show,  besides 
pronounced  negative  reactions  to  strong  or  injurious  me-  V 
chanical  stimulation,  what  he  called  a  sense  of  smell.  Its 
manifestations  depended  on  the  physiological  condition  of 
the  animal ;  that  is,  upon  its  degree  of  hunger.  If  kept  several 
days  without  food  a  starfish  would  immediately  perceive  its 
presence  and  crawl  toward  it.  "  Moreover,  if  a  small  piece  of 
the  food  were  held  in  a  pair  of  forceps  and  gently  withdrawn 
as  the  starfish  approached  it,  the  animal  could  be  led  about  the 
floor  of  the  tank  in  any  direction."  By  cutting  off  various 
parts  of  the  rays,  Romanes  found  that  "the  olfactory  sense 
was  equally  distributed  throughout  their  length;"  and  he 
also  showed  that  the  ventral  and  not  the  dorsal  surface  of  the 
body  was  concerned,  by  varnishing  the  latter,  which  left  the 
reactions  unaffected,  and  by  observing  that  when  a  bit  of  food 


82  The  Animal  Mind 

was  placed  on  the  back  it  remained  unnoticed  (365,  pp.  321- 
322).  Preyer  reported  great  individual  differences  in  the 
responses  of  starfish  to  food  stimulation ;  while  certain  speci- 
mens were  unmoved  by  the  neighborhood  of  food,  an  indi- 
vidual of  another  species  came  from  more  than  six  inches 
away  and  fell  upon  it  (350).  Whether  the  unlikeness  of 
behavior  was  due  to  the  species  difference  or  to  a  difference 
in  the  degree  of  hunger,  does  not  appear. 

§  23.     The  Chemical  Sense  in  Crustacea 

The  highest  invertebrate  animals  belong  to  the  phylum  of 
the  Arthropoda,  like  the  annelid  worms  in  their  segmented 
structure,  but  more  highly  organized  in  many  respects.  The 
body  of  a  typical  arthropod  consists  of  a  series  of  segments, 
one  behind  another,  each  segment  with  a  pair  of  appendages. 
The  higher  an  arthropod  stands  in  the  scale,  the  more  modi- 
fication and  differentiation  of  function  there  is  in  the  seg- 
ments and  appendages;  the  former  often  become  consoli- 
dated, and  the  latter  become  modified  for  swimming, 
walking,  or  sensory  purposes.  The  lowest  grand  division  of 
the  Arthropoda  is  that  of  the  Crustacea. 

As  the  animals  of  this  group  are  covered  with  a  hard  out- 
side shell,  sensitiveness  to  touch  and  chemical  stimulation  is 
ordinarily  referred  to  certain  hairs  scattered  over  the  body, 
and  to  the  modified  appendages  of  the  anterior  segments  which 
we  commonly  know  as  "feelers,"  the  large  and  small  antenna?. 
That  mechanical  contact  stimuli  in  certain  Crustacea  give  rise 
-JLto  specialized  reactions  is  evidenced  by  observations  on  the 
hermit  crab.  This  animal,  as  is  well  known,  has  acquired 
the  instinct  of  taking  up  its  abode  in  empty  shells,  most  com- 
monly those  of  some  gasteropod  mollusk.  When  wandering 
about  in  search  of  a  dwelling,  the  crab's  reactions  to  the  ob- 
jects it  meets  show  adaptation  to  the  character  of  the  stimulus, 


Sensory  Discrimination :  the  Chemical  Sense    83 

for  it  will  not  investigate  a  glass  tube  or  ball;  the  smooth 
surface  seems  not  to  be  the  adequate  stimulus  for  beginning 
the  movements  involved  in  exploring  and  entering  a  shell  (41). 
The  responses  of  Crustacea  to  food  stimulation  vary,  as 
might  be  expected,  with  different  genera  and  species.  Nagel 
finds  the  role  of  the  food  sense  in  aquatic  Crustacea  very  in- 
significant;  they  occasionally  show  antennal  movements  in 
the  presence  of  food,  he  says,  but  are  not  guided  to  it  (292). 
That  general  restlessness  is  shown  by  various  Crustacea  in  the 
neighborhood  of  food,  but  not  in  contact  with  it,  has  been  ob- 
served by  Bell  in  the  crayfish  (22),  by  Holmes  in  the  amphipod 
Amphithoe  longimana  (180),  by  Bateson  in  shrimps  and 
prawns  (n),  and  by  Bethe  in  the  green  crab  (28).  Bethe 
arranged  a  series  of  aquaria  one  above  the  other,  with  a  con- 
nection between  them,  and  found  that  when  food  was  placed 
in  the  uppermost  compartment  the  crabs  in  the  lower  ones 
were  successively  excited  as  the  food  juices  diffused  themselves 
from  each  compartment  to  the  one  below.  In  the  amphipod, 
the  small  antennae  and  the  mouth  parts  appeared  to  be  the 
regions  especially  sensitive  to  food  stimulation;  if  the  food 
touched  one  of  the  former,  the  animal  instantly  made  a  dart 
for  it.  Touching  the  antennule  with  a  needle  very  rarely 
caused  such  a  reaction  (180).  Bateson 's  shrimps  and  prawns 
had  their  food  sensibility  located  chiefly  in  the  antennules, 
though  if  food  was  placed  very  near  them  they  would  show 
disturbance  even  though  deprived  of  antennules  (n).  This 
was  the  case  also  with  Holmes's  amphipod.  Bell,  on  the  other 
hand,  found  the  whole  body  of  the  crayfish  sensitive  to  chemi- 
cal stimulation,  and  no  evidence  that  the  small  antennae  were 
especially  concerned.  The  crayfish's  reactions  to  contact  with 
food  were  such  as  to  direct  the  stimulus  toward  the  mouth ; 
negative  reactions  of  rubbing,  scratching,  and  pulling  at  the 
affected  part  were  obtained  by  stimulation  with  acids,  salts, 


84  The  Animal  Mind 

and  other  irritants  (22).  Evidences  of  irritation  by  the 
neighborhood  of  asafoetida  were  observed  also  by  Graber  in 
Pagurus  (152). 

In  some  Crustacea  the  sense  of  smell  is  possibly  concerned 
in  guiding  the  male  to  the  female.  Certain  copepods  which 
daily  migrate  from  near  the  surface  of  the  water  to  greater 
depths  and  back  again  have  had  this  behavior  explained  as  a 
result  of  the  reactions  of  the  females  to  light,  plus  the  tendency 
of  the  males  to  follow  the  females.  That  the  latter  is  an 
affair  of  chemical  stimulation  is  indicated  by  the  fact  that  the 
females  were  sought  even  when  concealed  hi  tubes  (304). 
In  the  case  of  some  other  Crustacea,  however,  the  sexes  do  not 
seem  to  be  aware  of  each  other's  neighborhood  until  they  come 
into  actual  contact  (182,  184). 

§  24.     The  Chemical  Sense  in  Arachnida 

The  two  most  important  divisions  of  the  phylum  Arthro- 
poda,  besides  the  Crustacea,  are  those  of  the  Arachnida  and 
Insecta.  Spiders,  as  is  well  known,  have  highly  developed 
^  responses  to  mechanical  stimulation ;  the  web-making  species 
in  particular  are  sensitive  to  very  slight  web  vibrations.  The 
food  reactions  of  spiders  have  never,  so  far  as  the  writer  knows, 
been  tested,  but  various  observers  report  sensitiveness  to 
chemical  stimulations,  such  as  those  produced  by  odorous 
oils,  not  in  contact  with  the  body.  Spiders  of  the  family 
Attidae  would  react  to  glass  rods  dipped  in  such  oils  and 
brought  close  behind  them,  but  would  not  react  to  clean  glass 
rods  when  similarly  placed  (320).  The  reactions  seem  to  be 
of  a  negative  character  (351),  and,  of  course,  in  all  such  cases 
it  remains  uncertain  whether  the  possible  conscious  accom- 
paniment is  a  specifically  olfactory  unpleasantness  or  an  un- 
pleasant irritation  of  the  body  surface.  Pritchett  found  that 
irritating  and  non-irritating  oils  gave  negative  reactions  (351) ; 


Sensory  Discrimination:  the  Chemical  Sense     85 

but  an  oil  that  belongs,  for  us,  to  the  latter  class  might  belong 
to  the  former  in  the  case  of  a  spider.  If  the  sensibility  were 
sharply  localized,  that  fact  would  point  in  the  direction  of  a 
specific  olfactory  sensation ;  but  while  some  authorities  think 
the  spider's  feelers  or  palpi  are  smell  organs  (25),  others 
believe  that  sensibility  to  chemical  stimulation  is  distributed 
over  the  body  (258,  351).  Nagel  finds  no  specific  organ  of 
smell  and  little  smell  sensibility  in  spiders  (292). 

A  member  of  the  Arachnida  which  presents  but  slight  super- 
ficial resemblance  to  the  spiders  is  Limulus,  the  horseshoe 
crab.  Limulus  shows  taste  reactions,  but  no  response  to 
smell  stimuli.  If  the  mandibles  at  the  base  of  the  legs  be 
rubbed  with  inedible  objects,  there  is  no  reaction.  Similar 
negative  results  are  obtained  by  holding  strong-smelling  food 
close  to  the  mouth  or  jaws.  But  if  an  edible  substance  be 
rubbed  on  the  mandibles,  strong  chewing  movements  take 
place.  Ammonia  or  acid  vapor  will  produce  these  same 
chewing  reflexes,  but  the  claws  make  snapping  movements 
"as  though  to  pick  away  some  disagreeable  object."  If  a  wad 
of  blotting  paper  wet  with  ammonia  or  acid  be  laid  on  the 
mandibles,  the  chewing  movements  are  reversed  and  the  object 
is  sometimes  picked  up  by  the  claws  and  removed.  Patten 
found  organs  which  he  believed  to  be  gustatory  on  both  the 
mandibles  and  the  claws  (315).  Pearl  observed  no  gustatory 
reactions  in  the  free-swimming  embryo  of  Limulus  (317). 

§  25.     The  Chemical  Sense  in  Insects 

Throughout  all  the  branches  of  the  animal  kingdom  thus  v 
far  mentioned,  the  chemical  sense  has  functioned  chiefly  as 
a  food  sense.  There  has  been  but  little  evidence jotJhe 
development  of  qualitative  discrimination  within  the  sense 
itself.  Thatis,  while  in  many  cases  an  animal  can  apparently 
distinguish  the  edible  from  the  inedible,  and  gives  negative 


s/ 


86  The  Animal  Mind 

reactions  to  irritating  chemicals,  one  would  hardly  be  jus- 
tified in  saying  that  it  possesses  more  than  one  food  sensation 
quality;  while  in  our  own  case,  of  course,  though  we  make 
comparatively  little  use  of  the  sense  of  smell,  the  qualitative 
discriminations  possible  by  its  means  are  many.  But  we 
come  now  to  a  group  of  animals  where  there  appears  a  remark- 
able  development  of  qualitative  variety  in  the  sensations 
resulting  from  chemical  stimulation;  namely,  the  Insecta. 
As  the  reactions  of  animals  to  mechanical  stimulation,  on  the 
other  hand,  offer  evidence  of  little  qualitative  difference  in 
the  accompanying  sensations,  we  shall  give  but  slight  atten- 
tion to  them  in  what  follows. 

To  begin  with,  there  is  evidence  that  taste  and  smell  are 
distinct  in  many  insects.  The  water  beetle  Dytiscus  margina- 
lis,  found  apparently  unresponsive  to  food  at  a  distance,  will 
bite  with  especial  eagerness  at  filter  paper  soaked  in  what 
Nagel  calls  "a  pleasant  solution"  (292).  Ants  fed  honey 
mixed  with  strychnine  will  taste  it  and  then  stop,  and  will  do 
this  even  when  the  antennae  and  mouth  palpi  are  removed, 
indicating  that  the  taste  organs  are  in  the  mouth  itself  (130). 
Similar  results  have  been  obtained  from  similar  tests  on  wasps, 
and  it  has  been  observed  that  wasps  so  treated  will  hesitate 
when  offered  pure  honey  afterward  (439). 

Vitus  Graber  tested  the  reactions  of  various  insects  to  odors 
by  the  method  which  we  called  on  page  60  the  Method  of  Pref- 
erence. This  was  Graber  's  favorite  mode  of  studying  the 
effect  of  stimuli  upon  animals.  Applied  to  olfactory  stimuli 
it  consisted  in  offering  a  choice  between  different  compart- 
ments, containing  each  a  different  odor.  The  animal's  power 
of  discrimination  was  argued  from  the  tendency  to  choose  cer- 
tain odors  rather  than  others.  Such  preferences  were  shown 
by  the  insects  (152).  The  method,  however,  as  was  noted 
above,  is  unsatisfactory,  because  discrimination  might  exist 


Sensory  Discrimination  :  the  Chemical  Sense    87 

where  preference  did  not.  Another  criticism  urged  against 
Graber's  experiments  is  that  the  odors  used  were  too  strong 
and  irritating.  The  insects  displayed  choice  between  odors 
even  when  their  antennae  were  removed ;  but  there  is  much 
evidence  to  show  that  the  antennae  are  the  true  organs  of  smell 
in  insects.  Various  flies  and  beetles  which  are  in  the  habit  of 
laying  their  eggs  in  putrefying  flesh  will  not  react  to  it  when 
their  antennae  are  removed,  and  it  has  been  shown  that  insects 
which  seem  to  find  their  mates  by  response  to  olfactory  stimu- 
lation, fail  to  do  so  when  deprived  of  antennae  (130).  Inter- 
esting "compensatory  movements"  have  been  seen  in  silk- 
worm moths  with  one  antenna  removed ;  they  turned,  that  is, 
in  the  direction  of  the  remaining  antenna  (219).  We  shall 
note  movements  of  this  class  later  in  insects  with  one  eye 
blackened,  and  in  fish  with  one  auditory  nerve  cut.  The 
exploring  movements  of  the  antennae  which  certain  insects 
make  in  seeking  a  proper  place  to  lay  their  eggs  in  have  been 
taken  as  evidence  of  the  smell  function  of  these  organs  (324). 
It  may  be,  then,  that  the  reactions  of  insects  without  antennae 
observed  by  Graber  were  due  rather  to  the  irritating  than 
to  the  properly  olfactory  character  of  the  stimuli. 

The  function  of  the  chemical  sense  in  the  mating  processes 
of  insects  is  one  of  the  most  remarkable  phenomena  connected 
with  the  sensory  reactions  of  animals.  Forel  says  he  had  a 
female  Saturnia  moth  shut  up  in  his  city  room,  and  that 
within  a  short  time  a  number  of  males  came  and  beat  against 
the  window  (130).  Riley  hatched  in  Chicago  some  moths 
from  the  Ailanthus  silkworm,  which  were  carefully  confined. 
No  other  specimens  were  known  to  exist  within  hundreds  of 
miles.  A  virgin  female  was  put  in  a  wicker  cage  on  an  ailan- 
thus  tree,  and  a  male,  with  a  silk  thread  tied  around  the  abdo- 
men for  identification,  was  liberated  a  mile  and  a  half  away. 
The  next  morning  the  two  were  together  (363). 


88  The  Animal  Mind 

The  most  interesting  observations  on  the  sense  of  smell 
as  used  in  the  mating  of  insects,  however,  are  those  of  Fabre. 
A  cocoon  of  the  "Bombyx  du  chene"  a  species  of  which  Fabre 
had  not  seen  a  specimen  in  the  locality  for  twenty  years,  was 
brought  to  him,  and  from  it  a  female  hatched.  Sixty  males 
sought  her  within  a  few  hours  after  she  reached  maturity. 
Fabre  noticed  in  this  and  other  cases  that  shutting  the  female 
in  an  air-tight  box  prevented  the  males  from  being  guided  to 
her,  but  that  the  smallest  opening  was  enough  to  allow  the 
odor  to  escape ;  that  the  males  were  not  in  the  least  confused 
or  led  astray  by  placing  dishes  of  odorous  substances  about, 
and  that  they  would  seek  anything  on  which  the  female  had 
rested  for  a  time,  a  fact  which  suggests  that  the  stimulus  is 
a  secretion  of  the  body,  as  it  is  known  to  be  in  silkworm  moths. 
Fabre  offers  the  suggestion  that  smell  stimuli  as  they  are  op- 
erative in  the  animal  kingdom  generally  may  be  of  two  classes : 

(1)  substances  which  give  off  particles  in  vapor  or  gas,  and 

(2)  substances  which  give  off  a  form  of  vibration.     Our  own 
olfactory  sense  is  limited  to  the  first  class  of  stimuli,  but  some 
animals,  notably  insects,  may  be  sensitive  to  both  (115). 
Certainly  the  marvellous  sensitiveness  involved  in  these  mating 
reactions  suggests  a  kind  of  response  to  stimulation  unknown 
in  human  experience. 

§  26.     How  Ants  find  Food 

In  many  ways  the  Hymenoptera  are  the  most  interesting 
of  insects,  particularly  those  members  of  the  order  which 
have  developed  community  life.  Their  reactions  to  chemical 
stimulation  have  been  the  subject  of  a  large  mass  of  literature, 
some  of  the  more  important  results  of  which  we  may  now 
undertake  to  survey,  considering  ants,  bees,  and  wasps  suc- 
cessively. Sir  John  Lubbock  was  among  the  earliest  observ- 
ers to  indicate  the  great  importance  of  chemical  stimuli 


Sensory  Discrimination :  the  Chemical  Sense    89 

in  the  life  of  ants.  In  the  first  place,  he  demonstrated  that  it 
is  by  chemical  stimulation  that  ants  are  able  to  follow  each  tf 
other  to  supplies  of  food;  or  to  larvae,  for  an  ant's  behavior  to 
an  ant  larva  found  in  the  course  of  its  wandering  is  like  its 
behavior  to  food;  the  larva  is  picked  up  and  carried  to  the 
nest.  Lubbock  put  some  larvae  on  a  glass  plate  at  a  little 
distance  from  one  of  his  artificial  ant  nests,  and  set  a  similar 
empty  plate  beside  it;  he  then  made  a  bridge  of  a  strip  of 
paper  leading  from  the  nest  toward  the  plates,  and  connected 
each  of  them  with  this  bridge  by  a  separate  short  paper  strip. 
He  placed  a  marked  ant  at  the  larvae ;  she  picked  up  one  and 
returned  to  the  nest.  She  soon  appeared  followed  by  several 
others ;  when  she  had  reached  the  larvae,  and  before  the  others 
had  arrived  at  the  dividing  of  the  ways,  Lubbock  exchanged 
the  short  strips,  so  that  the  one  over  which  the  marked  ant 
had  passed  now  led  to  the  empty  plate.  The  following  ants 
all  took  this  path,  indicating  that  they  were  guided  by  some 
trace  which  her  footsteps  had  left.  Lubbock  was  inclined 
to  think,  however,  that  some  kind  of  communication  must 
have  passed  between  the  marked  ant  and  her  fellows  in  the 
nest  to  induce  them  to  follow  her,  and  also  that  this  communi- 
cation might  on  occasion  convey  some  notion  of  the  quantity 
of  food  or  larvae  to  be  had.  He  placed  three  glass  plates  near 
an  ant  nest,  connecting  each  of  them  with  the  nest  by  means 
of  a  paper  strip.  On  one  plate  he  put  a  heap  of  several  hun- 
dred larvae,  on  the  second  two  or  three  only;  the  third  was 
empty.  He  put  marked  ants  on  each  of  the  plates,  and  cap- 
tured all  the  ants  which  they  led  back  with  them.  Many 
more  ants  came  to  the  plate  with  the  larger  heap  of  larvae 
than  to  the  others.  Lubbock  explained  this  by  supposing 
that  the  ant  from  that  dish  had  in  some  way  communicated 
to  the  nest  the  greater  numbers  at  her  disposal  (248,  pp. 
172  ff.).  Obviously  it  would  be  enough  to  suppose  that  the 


QO  The  Animal  Mind 

smell  of  food  or  larvae  about  an  ant  returning  laden  to  the 
nest  is  a  stimulus  to  her  nestmates  to  follow  her;  that  this 
smell  is  stronger,  the  larger  the  stock  she  has  found,  and 
hence  acts  as  a  more  powerful  stimulus. 

§  27.    How  Ants  find  the  Way  Home 

Lubbock's  experiments  indicated  also  that  in  finding  their 
way  back  to  the  nest  ants  make  more  use  of  smell  than  of  sight. 
One  only  of  these  observations  need  be  described.  Lub- 
bock  placed  larvae  in  a  dish  on  a  table  connected  by  a  bridge 
with  an  ant  nest.  He  accustomed  the  ants  to  go  back  and 
forth  from  the  dish  to  the  nest  along  a  path  which  he  diversi- 
fied by  artificial  scenery,  such  as  rows  of  bricks  along  either 
side,  and  a  paper  tunnel.  When  the-  path  was  thoroughly 
learned  he  moved  the  bricks  and  the  tunnel  so  that  they  led 
in  a  different  direction;  the  ants,  however,  were  not  at  all 
disconcerted  by  this  cataclysm  of  nature,  but  followed  the 
same  track  as  before,  evidently  guided  by  their  own  footprints 
(248,  p.  259).  The  direction  of  the  light  is  not  without  some 
influence,  however.  When  two  candles  that  had  stood  near 
the  nest  were  moved  to  the  opposite  side,  some  of  the  ants 
were  confused  (248,  p.  268).  Bethe,  whose  object  is  to  show 
that  all  ant  behavior  is  a  series  of  unconscious  reflexes  to 
chemical  stimuli,  made  the  following  attempt  to  study  the 
formation  of  a  new  ant  path.  He  placed  near  the  entrance 
to  a  nest  a  large  sheet  of  paper  covered  with  lampblack,  on 
which  the  footsteps  of  the  ants  could  be  traced.  On  this  paper 
he  placed  a  supply  of  food.  When  an  ant  had  found  the  food, 
in  going  back  to  the  nest  she  always  followed  the  path  by 
which  she  had  come,  except  that  when  the  original  path  had 
crossed  itself  in  loops,  the  ant  omitted  the  loops  in  returning. 
Other  ants  followed  the  same  path,  though  they  all  had  a  ten- 
dency to  cut  off  curves  (30).  Wasmann,  the  ardent  opponent 


Sensory  Discrimination  :  the  Chemical  Sense    91 

of  Bethe's  reflex  theory,  looks  with  suspicion  on  this  tendency 
gradually  to  straighten  the  path,  and  thinks  an  animal  re- 
flexly  drawn  along  by  a  chemical  stimulus  on  the  ground 
would  make  no  improvements  in  the  route  (426).  The  evi- 
dence that  it  is  the  sense  of  smell,  if  not  a  smell  reflex,  that 
guides  the  ants  remains,  however,  strong.  Bethe  further 
points  out  that  the  chemical  stimulus  deposited  by  the  feet 
of  the  ants  is  volatile.  If  a  strip  of  paper  be  placed  across 
an  ant  path,  the  ants  on  coming  to  it  stop,  quest  about,  and 
are  delayed  until  one  accidentally  runs  across  the  strip  and 
others  follow.  (Why,  asks  Wasmann,  if  they  are  being  re- 
flexly  drawn  along,  do  they  not  merely  stop  short  when  the 
stimulus  fails,  instead  of  hunting  for  it  ?)  The  piece  of  paper 
is  thus  gradually  adopted  into  the  ant  road;  if  it  is  subse- 
quently removed,  the  ants  stop  and  are  bewildered  at  the 
place  where  it  was,  showing  that  the  earlier  traces  of  their 
footsteps,  under  the  paper,  have  evaporated.  Again,  Bethe 
thinks  he  has  evidence  that  the  chemical  stimulus  left  by  the 
feet  of  ants  going  from  the  nest  is  different  from  that  deposited 
by  those  going  to  the  nest,  and  that  ants  on  the  way.  home 
will  not  follow  a  track  made  by  the  feet  of  other  ants  on  the 
outward  journey,  and  vice  versa  (30).  That  they  will  follow 
their  own  individual  track  in  either  direction  is  shown  by 
the  smoked  paper  experiment  just  described,  and  also  by  an 
experiment  of  Fielde's,  where  ants  finding  their  way  through 
a  labyrinth  from  a  heap  of  pupae  to  the  nest  and  back 
again  followed  each  one  its  own  trail  without  regard  to  the 
others  (218).  Bethe  found  that  when  the  usual  road  to  an 
ant  nest  had  been  interrupted  by  the  removal  of  a  heap  of 
sand,  and  the  road  across  the  breach  had  been  established 
solely  by  incoming  ants,  the  outgoing  ants  refused  to  follow 
it,  and  made  a  new  road  for  themselves  (30).  Wasmann 
thinks  this  may  have  been  done  merely  on  account  of  the 


92  The  Animal  Mind 

faintness  of  the  recently  established  path  as  compared  with 
the  old  one  (426).  Bethe  observed  also  that  if  a  strip  of  paper 
had  been  adopted  into  an  ant  road,  and  was  then  while  an 
ant  was  on  it  rotated  through  180  degrees,  the  ant  stopped 
and  was  disturbed  on  coming  to  the  end  of  it  (30).  Experi- 
ments on  rotating  ants  were  made  also  by  Lubbock  (248), 
and  seem  to  give  puzzling  and  conflicting  results;  it  is  not 
clear  why  even  on  the  assumption  that  there  is  a  difference 
in  odor  between  the  road  to  the  nest  and  that  from  the  nest, 
an  ant  on  a  road  which  led  both  ways  should  have  found  her 
course  interrupted  by  rotation.  One  fact,  Bethe  thinks, 
shows  that  even  assuming  two  road  smells  is  not  enough. 
Ants  of  certain  families  (Lasius)  which  habitually  make  regu- 
lar and  frequented  roads,  can  if  they  come  upon  one  of  these 
roads  in  wandering  at  once  take  the  proper  direction,  either 
to  or  from  the  nest.  Evidently  the  mere  presence  of  two 
smells  would  not  enable  them  to  do  this.  Bethe  suggests 
that  the  particles  of  the  two  chemical  substances  are  also 
differently  polarized,  so  that  one  of  them  can  be  followed  only 
in  one  direction,  the  other  in  the  opposite  direction  (30). 
Wasmann  objects  to  this  that  an  ant  returning  on  its  own  traces 
would  destroy  them,  as  the  opposite  polarizations  would  can- 
cel; and  that  similar  confusion  would  occur  on  a  narrow 
and  much  frequented  road  (426).  He  and  Forel  (132)  both 
think  that,  granting  the  distinction  between  the  outward 
and  inward  paths,  which  is  made  by  only  a  few  families  of 
ants,  the  direction  is  most  probably  given  by  a  perception  of 
the  "smell  form"  of  the  footsteps  obtained  through  the 
antennae. 

Recently  C.  H.  Turner  has  come  to  the  conclusion  that  ants 
are  not  guided  " slavishly"  or  reflexly  by  the  odor  of  their 
tracks  in  finding  the  way  to  and  from  the  nest.  He  made 
a  small  cardboard  stage  from  which  an  inclined  cardboard 


Sensory  Discrimination:  the  Chemical  Sense     93 

bridge  led  down  to  the  artificial  ant  nest.  Ants  and  pupae 
were  placed  on  the  stage.  After  the  ants  had  through 
random  movements  learned  the  way  down  the  incline 
a  second  incline  was  placed  so  as  to  lead  from  the  opposite 
side  of  the  stage  to  the  nest.  No  ants  went  down  this  way. 
The  inclines  were  then  exchanged  so  that  the  one  bearing  the 
scent  of  the  ants'  footprints  was  on  the  opposite  side  and  the 
unscented  incline  in  the  old  place ;  the  ants  continued  to  go 
down  in  the  old  place.  It  is  unsafe  to  criticise  an  experiment 
without  having  actually  seen  it,  but  Turner's  account  does  not 
exclude  the  possibility  that  the  ants  were  guided  in  setting 
out  on  their  homeward  course  by  the  scent  of  their  footprints 
on  the  cardboard  stage,  which  seems  to  have  remained  un- 
changed. He  confirms  Bethe's  observation  that  the  path- 
ways to  and  from  the  nest  are  different,  but  does  not  find 
that  even  a  single  ant  follows  her  own  footsteps  in  both  di- 
rections. The  direction  of  the  light,  not  smell,  is  the  ruling 
factor  in  pathfinding,  according  to  Turner,  who  offers  the 
following  experimental  evidence.  When  the  stage  with  the 
first  incline  was  arranged  as  before  and  a  16  c.p.  lamp 
placed  near  the  side  to  which  the  incline  was  attached,  the  ants 
learned  to  go  down  the  pathway  to  the  nest.  When  the  in- 
cline on  the  opposite  side  was  added,  and,  after  waiting  a  time 
to  make  sure  that  no  ants  went  down  that  way,  the  lamp 
was  moved  to  the  other  side,  marked  disturbance  was  shown 
by  the  ants.  "In  most  cases,  some  would  finally  go  down 
the  new  incline;  in  a  few  cases  after  the  lapse  of  several 
minutes  all  went  down  the  new  incline."  Altering  the  in- 
tensity of  the  light  had  no  effect ;  the  disturbance  was  caused 
by  any  decided  change  in  its  direction  (408). 

In  all  probability,  different  species  of  ants  vary  in  the 
degree  to  which  they  make  use  of  smell  as  a  guide.  Pieron, 
for  example,  finds  that  Formica  cinerea  depends  more  upon 


94  The  Animal  Mind 

vision  than  Lasius  fuliginosus,  which  is  guided  largely  by 
smell  (325),  and  the  same  conclusion  is  reached  by  Was- 
mann  (426).  Pieron  calls  attention  to  still  another  factor 
which  he  calls  muscular  memory,  influential  in  all  the  species 
he  tested,  but  especially  so  in  Aph&nogaster  barbara  nigra. 
If  an  ant  of  this  species  is  returning  to  the  nest  and  steps  on 
a  bit  of  paper  covered  with  earth,  she  may  be  moved  bodily 
to  some  distance  without  seeming  to  notice  the  fact.  In  such 
a  case,  on  being  set  down,  she  continues  her  march  "in  the 
new  region,  following  a  direction  absolutely  identical  with  that 
which  she  was  following  in  her  return  to  the  nest,  and  she  does 
not  stop  or  seem  disturbed  until  after  a  more  or  less  prolonged 
march,  often  about  equal  to  the  distance  that  separated  her, 
at  the  moment  when  she  was  displaced,  from  the  entrance  to 
the  nest."  Even  if  her  displacement  occurs  near  the  entrance 
to  the  nest,  she  will  go  on  past  it  and  stop  at  a  distance  about 
equal  to  that  which  she  would  ordinarily  have  had  to  traverse. 
Evidently  smell  is  not  here  concerned  (325).  The  ant  would 
seem  to  be  like  a  little  machine  wound  up  to  go  just  so  far 
and  to  take  just  such  turnings.  We  shall  mention  this 
observation  again  in  a  later  chapter. 

§  28.    How  Ants  "recognize"  Nest  Mates 

Another  problem  of  ant  life  to  which  smell  appears  to 
furnish  the  key  is  that  of  the  recognition  of  nest  mates.  It  has 
long  been  known  that  an  ant  entering  a  strange  nest,  though 
of  the  same  species,  is  likely  to  meet  with  rough  treatment, 
and  even  be  put  to  death.  Now  Forel  found  in  1886  that 
ants  of  the  genus  Myrmica  whose  antennae  were  removed 
would  attack  their  own  nest  mates  (130).  It  seems  probable 
that  each  nest  of  ants  has  a  peculiar  odor  which  is  the  basis 
of  the  distinction  between  friends  and  foes.  Bethe  tested  the 
smell  theory  by  dipping  an  ant  first  in  weak  alcohol,  then 


Sensory  Discrimination  :  the  Chemical  Sense     95 

in  water,  and  then  in  the  juices  obtained  by  crushing  the 
bodies  of  a  number  of  ants  of  another  species.  He  found 
that  an  ant  thus  treated  would  be  attacked  and  killed  by 
its  own  nest  mates,  but  could  be  introduced,  though  not 
so  easily,  into  the  nest  whose  odor  it  now  presumably  bore, 
even  though  its  appearance  was  quite  different  from  that 
of  the  ants  therein  (30).  Wasmann  repeated  these  experi- 
ments with  much  less  success  than  Bethe ;  bathing  Myrmica 
ants  with  essence  of  Tetramorium  ant  did  not  preserve 
them  from  final  destruction  at  the  jaws  of  the  latter, 
though  it  delayed  their  fate;  nor  did  much  bathing  with 
foreign  nest  odors  induce  their  nest  mates  to  attack  ants 
of  the  species  Lomechusa  strumosa,  though  they  seemed 
disturbed  at  first.  Wasmann  apparently  thinks  other  fac- 
tors besides  smell,  vision  perhaps,  enter  into  the  recogni- 
tion process  (426).  Bethe,  in  a  later  paper,  suggests  that 
Wasmann's  negative  results  may  have  been  due  to  the  fact 
that  the  nest  smell  very  quickly  returns  to  the  ants  after 
it  has  been  removed ;  he  himself  took  account  only  of  the 
first  reaction  of  other  ants  toward  the  one  subjected  to 
treatment  (31). 

Fielde,  as  the  result  of  a  study  of  the  genus  Stenamma, 
concludes  that  each  ant  is  the  bearer  of  three  distinct  odors : 
the  individual  odor,  which  enables  her  to  follow  her  own 
trail  in  a  labyrinth,  and  the  reception  of  which  depends  upon 
the  tenth  segment  of  the  antennae ;  the  race  odor,  depend- 
ent on  the  eleventh  segment;  and  the  nest  odor,  depend- 
ent on  the  twelfth  (118).  In  a  later  article  she  concludes 
that  the  nest  odor  of  the  worker  ants  is  derived  from  their 
queen  mother;  that  the  odor  of  the  queen  is  unchanging, 
and  is  imparted  to  her  eggs.  The  worker,  however,  gradually 
changes  its  odor.  Queens  of  diverse  odors  may  be  produced 
by  the  influence  of  males  that  are  the  offspring  of  worker 


96  The  Animal  Mind 

mothers  and  have  the  differentiated  worker  odor.  A  young 
ant  isolated  from  the  pupa  stage  until  many  days  old  will 
single  out  its  queen  mother  from  queens  of  other  species, 
but  will  show  decided  suspicion  of  older  sister  worker  ants. 
A  mixed  nest  formed  of  newly  hatched  ants  of  different 
species  was  separated  for  seven  months.  On  rejoining  each 
other,  the  ants  showed  hostility ;  their  odor,  Fielde  argues, 
had  changed.  But  young  ants  of  one  species  were  received 
by  those  of  the  other  species.  Fielde  does  not  hesitate  to 
introduce  the  psychic  factor  and  say  that  the  latter  remem- 
bered the  odor  of  the  young  ones,  having  been  associated  with 
it  in  their  own  youth.  The  suggestion  might  be  made  that 
the  young  ants  had  not  as  yet  developed  any  specific  odor, 
but  this  is  opposed  by  the  observation  that  newly  hatched 
Lasius  ants  from  a  strange  colony  were  not  received  by  a  nest 
of  Stenammas,  while  young  Lasius  ants  from  a  colony  with 
which  the  Stenammas  had  been  acquainted  in  youth  were 
accepted  eleven  months  after  the  latter  had  been  segregated. 
It  is  an  affair  of  the  memory,  Fielde  is  assured ;  and  she  says, 
"If  an  ant's  experience  be  narrow,  it  will  quarrel  with  many, 
while  acquaintance  with  a  great  number  of  ant  odors  will 
cause  it  to  live  peaceably  with  ants  of  diverse  lineage,  pro- 
vided the  odors  characterizing  such  lineage  and  age  environ 
it  at  its  hatching"  (123).  Bethe  held  that  an  ant's  own  nest 
odor  offered  no  stimulus  to  it  at  all,  but  that  fighting  reflexes 
were  occasioned  by  any  foreign  nest  odor  (30).  Many  facts, 
however,  seem  to  tell  against  this  view;  among  others,  the 
early  observation  of  Forel  that  a  Myrmica  ant  deprived  of 
its  antennae  attacks  everything  in  sight  (130).  It  should, 
according  to  Bethe's  theory,  live  peaceably  with  all. 

Thus  we  see  that  in  spite  of  some  divergence  of  testimony, 
there  is  evidence  that  ants  have  a  variety  of  qualitatively 
different  smell  experiences :  the  smell  of  food  and  of  larvae, 


Sensory  Discrimination :  the  Chemical  Sense     97 

probably  distinct,  though  there  is  no  experimental  proof  of 
the  fact;  the  individual  smell  of  an  ant's  own  footsteps; 
a  possible  distinction,  in  some  species,  between  the  smell  of 
the  outgoing  and  that  of  the  incoming  paths ;  and  the  different 
odors  which  seem  to  be  responsible  for  the  discrimination  be- 
tween nest  mates  and  foreigners.  If  it  is  true,  as  Fielde 
maintains,  that  loss  of  the  eighth  and  ninth  segments  of  the 
antennae  renders  an  ant  incapable  of  caring  for  the  young, 
then  the  recognition  of  larvae  and  pupae  does  depend  upon  a 
specific  odor  (118).  Forel  makes  an  interesting  distinction 
between  the  sense  of  smell  in  insects  with  immovable  anten- 
nae and  the  same  sense  where  the  antennae,  as  in  ants,  can 
be  moved  about  over  objects.  In  the  former  case  it  is  as 
with  us  a  qualitative  sense  pure  and  simple,  giving  informa-  *., 
tion  of  objects  at  a  distance ;  in  the  latter  case  it  is  a  contact 
sense,  and  may  give  rise  to  spatial  as  well  as  qualitative  per- 
ceptions. He  compares  the  antennae  to  a  pair  of  olfactory 
hands,  and  points  out  how  such  organs  may  allow  of  the  per- 
ception of  the  " smell  form"  of  objects  (132). 

§  29.    How  Bees  are  attracted  to  Flowers 

In  bees  the  sense  of  smell  is  equally  well  developed.  But 
no  topic  in  comparative  psychology  has  been  more  hotly  . 
disputed  than  the  use  which  bees  make  of  this  sense,  and  the  * 
extent  to  which  they  depend,  rather,  upon  sight.  Darwin 
(90)  and  H.  Muller  (284,  285)  thought  both  color  and 
fragrance  influential  in  attracting  insects.  Plateau  main- 
tains that  the  chief  influence  attracting  bees  to  flowers  is 
smell,  and  that  color  has  little  effect.  He  made  a  number  of 
experiments  in  which  the  brightly  colored  corollas  of  flowers 
were  cut  off  without  disturbing  the  nectaries,  and  claims  to 
have  found  that  the  visits  of  bees  to  the  mutilated  flowers 
were  as  frequent  as  before  (336-338,  339,  341).  On  the  other 


98  The  Animal  Mind 

hand,  Giltay  obtained  opposite  results;  the  flowers  whose 
corollas  were  removed  were  neglected  by  bees,  while  those 
which  were  covered  so  as  to  be  invisible,  but  not  so  as  to 
prevent  the  odor  from  escaping,  were  also  unnoticed  (144). 
Josephine  We*ry  found  that  the  proportion  of  bees  visiting 
flowers  with  intact  corollas  to  those  visiting  flowers  with  the 
corollas  removed  was  66 : 18  (434).  Kienitz-Gerloff  criticises 
Plateau's  figures  and  the  accuracy  of  his  experiments  (220). 
Forel  found  that  a  bee  with  the  antennas  and  all  the  mouth 
parts  removed,  hence  probably  incapable  of  smell,  returned 
to  flowers  for  honey,  though  of  course  without  success  (130). 
Andreae  thinks  that  among  diurnal  insects  those  which  live 
on  the  ground,  and  take  but  short  flights,  are  more  influenced 
by  smell ;  while  the  freely  flying  insects  are  attracted  by  the 
sight  of  flowers  (5). 

§  30.    How  Bees  find  the  Hive 

Most  complicated  of  all  is  the  problem  as  to  how  bees  find 
their  way  back  to  the  hive.  It  is  obvious  that  the  simple  ant 
method  of  following  a  chemical  trail  is  ruled  out  for  in- 
sects that  fly.  Bethe  abandons  the  puzzle  as  insoluble  (30). 

Von  Buttel-Reepen  attempts  at  length,  and  with  a  vast 
amount  of  apic  lore,  to  refute  his  position.  It  would  be  im- 
possible to  give  more  than  the  briefest  statement  of  the  argu- 
ments of  both  sides.  Bethe  maintains  that  the  smell  of  the 
hive  does  not  guide  the  bees  back  to  it,  because  he  found 
that  if  the  hive  were  rotated  slowly  enough  to  allow  the  cloud 
of  nest  smell  at  the  opening  to  move  with  the  opening,  the  bees 
returning  would  not  follow  it  for  more  than  45°,  but  would 
go  to  the  place  where  the  opening  had  been.  He  thinks 
they  are  not  guided  by  sight,  because  when  he  completely 
changed  the  appearance  of  the  hive,  masking  it  with  branches, 
and  other  coverings,  the  bees  were  not  disconcerted,  but  flew 


Sensory  Discrimination :  the  Chemical  Sense     99 

straight  to  the  mouth  of  the  hive.  He  brings  other  evidence 
against  the  vision  hypothesis  which  we  shall  discuss  in 
Chapter  XI.  An  unknown  force,  he  concludes,  guides 
the  bee  in  its  homing  flight  (30).  Von  Buttel-Reepen 
believes  that  visual  memory  will  explain  all  the  facts; 
that  the  bees  were  not  disturbed  by  the  altered  appearance 
of  their  hive  because  they  knew  their  way  so  thoroughly 
that  nothing  could  disturb  them  by  the  time  they  had 
come  so  nearly  home.  The  visual  memory  required  is,  he 
admits,  of  a  peculiar  sort,  which  we  shall  consider  in  a  later 
chapter.  The  odor  of  the  hive  does  cooperate  with  vision  in 
certain  cases ;  when  a  stock  of  bees  has  been  moved  without 
their  knowledge,  they  fly  out  without  making  any  "  orienting 
flight,"  as  they  commonly  do  on  leaving  a  new  place,  a  fact 
that  is  one  of  the  evidences  for  the  visual  memory  theory. 
Nevertheless,  many  of  them  succeed  in  finding  their  way  back, 
and  then,  if  their  hive  is  placed  among  a  number  of  others, 
von  Buttel-Reepen  thinks  they  " smell"  their  way  back  to 
the  right  one.  He  mocks  at  Bethe's  unknown  force,  on  the 
ground  that  it  must  sometimes  lead  the  bee  to  the  hive  and 
sometimes  back  to  the  place  where  food  has  been  found  (72). 
Bethe  attempts  to  answer  this  by  saying  that  the  force  acts 
in  cooperation  with  the  physiological  condition  of  the  animal ; 
the  laden  bee  follows  it  to  the  hive,  the  bee  with  the  empty 
crop  is  led  back  to  the  food  supply  (32).  Of  course  one  may 
say  what  one  pleases  about  the  modus  operandi  of  an  unknown 
force  without  fear  of  disproof,  but  also  without  carrying  much 
conviction. 

§ .31.    How  Bees  "recognize"  Nest  Mates 

The  nest  smell,  which  characterizes  each  hive  and  prevents 
the  reception  of  strangers,  who  are  treated  precisely  after  the 
fashion  of  ants  in  similar  circumstances,  is  composed  accord- 


ioo  The  Animal  Mind 

ing  to  von  Buttel-Reeperi  of  the  following  odors :  the  indi- 
vidual odor  of  different  workers ;  the  family  odor,  common 
to  all  the  offspring  of  the  same  queen;  the  larval  smell  and 
food  smell;  the  drone  smell,  the  wax  smell,  and  the  honey 
smell.  There  are  various  ways  in  which  the  mode  of  reaction  to 
a  foreign  nest  smell  is  modified.  If  two  bee  stocks  are  placed 
side  by  side,  and  one  has  the  queen  and  entire  brood  removed, 
it  will  go  over  to  the  other  stock  and  be  kindly  received.  One 
can  understand  that  the  attraction  of  the  queen  and  brood  odor 
may  overcome  the  tendency  of  the  foreign  nest  smell  to  repel 
the  invaders,  but  it  is  harder  to  see  why  the  more  fortunate 
stock  should  allow  itself  to  be  invaded.  Further,  a  bee  laden 
with  honey  can  get  itself  received  by  a  foreign  stock  that  has 
exchanged  hives  with  it,  where  an  unladen  bee  is  attacked; 
here  the  smell  of  the  honey  may  overcome  the  foreign  smell. 
As  is  well  known,  two  alien  stocks  may  be  united  by  sprin- 
kling them  with  some  odorous  substance.  The  queen  odor 
is  the  strongest  factor  in  the  nest  smell ;  in  swarming  it  over- 
comes the  tendency  to  return  to  the  old  nest,  and  queen- 
less  swarms  will  join  themselves  to  foreign  swarms  having  a 
queen.  The  apparent  attention  paid  to  the  queen  while 
laying  eggs,  the  gathering  of  workers  around  her  trilling  their 
antennae  toward  her,  suggest  strongly  that  her  odor  is 
pleasant  to  them.  The  queen  herself,  however,  is  perfectly 
indifferent  to  any  foreign  nest  smell,  and  will  beg  food  of  any 
bee,  even  those  which  are  angrily  crowded  around  her  cage 
in  a  foreign  hive.  Drones  also  will  go  from  stock  to  stock, 
and  are  always  peacefully  received  until  drone-killing  time 
begins.  It  has  usually  been  supposed  that  the  unrest  dis- 
played by  a  bee  stock  when  deprived  of  its  queen  is  due  to  the 
absence  of  the  queen  odor,  and  it  seems  almost  certain  that 
this  must  be  a  powerful  influence,  though  von  Buttel-Reepen 
thinks  it  is  not  the  only  influence,  for  he  has  observed  that  if 


Sensory  Discrimination :  the  Chemical  Sense    101 

the  queen  be  replaced  in  the  honey  space,  removed  from  the 
rest  of  the  hive,  the  bees  will  quiet  instantly,  before  the  smell 
has  had  time  to  diffuse  itself.  Also,  bees  sometimes  behave 
as  if  they  had  lost  their  queen  when  she  is  only  put  in  a  cage, 
and  her  odor  is  perfectly  accessible  (72). 

It  is  clear  that  bees  as  well  as  ants  are  capable  of  dis-  >*/ 
tinguishing  a  considerable  number  of  smell  qualities.  Prob- 
ably  the  same  thing  is  true  of  the  social  wasps.  In  the 
solitary  wasps,  however,  we  find  less  evidence  of  a  highly 
developed  sense  of  smell,  or  rather  of  a  great  variety  of  smell 
reactions,  and  the  solitary  bees  are  very  likely  less  influenced 
by  smell  than  the  social  bees.  In  the  interesting  study  of  the 
solitary  wasps  by  Mr.  and  Mrs.  Peckham,  it  appears  that 
sight  plays  a  far  more  important  role  than  smell  for  these 
insects  and  the  return  to  the  nest  in  particular  seems  to  be 
almost  entirely  an  affair  of  sight  (322,  323).  In  general 
the  greatest  development  of  qualitative  variety  in  the  sense  of 
smell  is  found  in  the  social  Hymenoptera,  and  is  probably 
a  product  of  the  social  state.  Perris,  however,  noted  that 
the  solitary  wasp  Dinetus  was  much  disturbed  in  finding  its 
nest  hole  if  he  had  placed  his  hand  over  the  hole  during 
the  wasp's  absence,  and  thought  the  odor  of  his  hand  was 
distracting  to  the  insect  (324). 

§  32.    The  Chemical  Sense  in  Vertebrates 

Although  the  vertebrates  stand  at  the  head  of  the  animal 
kingdom,  yet  in  point  of  complexity  of  structure  and  behav- 
ior the  lowest  vertebrate  is  far  below  the  highest  members  p 
of  the  invertebrate  division.  When  we  undertake  to  study 
the  responses  to  special  stimulation  displayed  by  this 
same  lowest  vertebrate,  the  little  Amphioxus  or  lancelet, 
it  is  like  going  back  to  the  earthworm.  The  only  kind  of 
evidence  that  contact,  chemical,  and  temperature  stimuli 


IO2  The  Animal  Mind 

produce  specific  sensation  qualities  is  found  in  the  fact  that 
^sensibility  to  them  is  differently  localized,  and  may  be  in- 
dependently fatigued.  To  weak  acid,  the  head  end  of  the 
animal  is  most  sensitive,  the  posterior  end  less,  the  middle 
least ;  to  contact  with  a  camel's-hair  brush,  the  two  ends  are 
equally  sensitive  and  more  so  than  the  middle;  to  a  current 
of  warm  water  the  order  of  sensitiveness  is  head  end,  mid- 
dle, posterior  end  (311). 

For  fishes,  as  for  all  aquatic  animals,  the  distinction  be- 
V-  tween  smell  and  taste  becomes  obscure.  The  neighborhood 
of  food  not  in  actual  contact  with  the  body  seems  to  stir  fish 
to  activity,  but  not  to  direct  their  movements.  Bateson 
(12)  and  Herrick  (165)  both  obtained  evidence  of  this;  Nagel, 
on  the  other  hand,  declares  that  fish  do  not  perceive  food  at  a 
distance  except  by  sight,  and  that  the  function  of  the  first  pair 
of  cranial  nerves  in  these  animals  must  remain  uncertain  (292). 
The  well-developed  character  of  these  " olfactory"  nerves 
and  lobes,  whose  function  in  higher  vertebrates  is  certainly 
connected  with  smell,  would  argue  against  the  supposition  that 
smell  can  be  wholly  lacking  in  fishes.  It  is  generally  agreed 
that  a  contact  food  sense  exists  in  fish;  Nagel,  however, 
holds  that  its  organs  are  situated  only  about  the  mouth  (292), 
while  Herrick  has  good  experimental  proof  that  fishes  which 
have  "terminal  buds,"  structures  resembling  taste  buds, 
distributed  over  the  skin,  are  also  sensitive  to  food  stimulation 
applied  to  different  regions  of  the  skin.  He  thinks  that  Nagel's 
negative  results  were  due  to  the  fact  that  instead  of  food  stimuli 
in  his  experiments  he  used  chemicals  with  which  the  fish 
would  not  normally  be  acquainted  (165).  Nagel  thinks  the 
role  of  the  chemical  sense  in  Amphibia  also  is  negligible  (292), 
and  there  is  no  experimental  evidence,  to  the  writer's  knowl- 
edge, indicating  specific  taste  or  smell  sensations  either  in  Am- 
phibia or  in  reptiles.  In  birds  the  high  development  of  both 


Sensory  Discrimination:  the  Chemical  Sense    103 

sight  and  hearing,  and  the  fact  that  almost  all  reactions  are 
made  in  response  to  the  stimuli  for  these  senses,  masks  the  ^ 
presence  of  olfactory  sensitiveness  if  it  exists.  Taste,  birds 
seem  to  have ;  the  chicks  experimented  on  by  Lloyd  Morgan, 
for  example,  displayed  disgust  at  picking  up  bits  of  orange 
peel  instead  of  yolk  of  egg  (281,  pp.  40-41).  Xavier  Raspail 
is,  so  far  as  the  writer  knows,  the  only  observer  who  has  ex- 
pressed a  definite  opinion  in  favor  of  the  sense  of  smell  in 
birds.  He  thinks  they  abandon  eggs  that  have  been  handled 
because  they  detect  the  fact  by  smell ;  that  they  find  buried 
grubs  by  smell,  and  are  guided  by  this  sense  to  concealed  food 
and  water.  The  last  statement  he  supports  by  the  observa- 
tion that  their  tracks  lead  straight  to  hidden  food  on  their 
first  visit  to  it,  showing  that  it  was  not  found  by  accident 

(359)- 

When  we  come  to  the  Mammalia,  however,  we  find  in  the 
great  majority  of  types  a  very  high  development  of  qualitative  V" 
discrimination  in  the  sense  of  smell.  Hunters  know  it  to 
be  the  chief  defensive  weapon  of  wild  animals,  and  it  has 
retained  great  keenness  in  many  domesticated  ones,  —  the 
cat,  for  instance,  which  will  be  awakened  from  slumber  in 
the  garret  by  the  odor,  quite  unsuspected  of  human  nostrils, 
of  some  favorite  food  being  prepared  in  the  kitchen,  and  is 
thrown  into  ecstasy  at  a  faint  whiff  of  catnip.  The  dog,  how- 
ever, is  the  hero  of  this  field  of  mental  prowess.  The  ex- 
periments of  Romanes  on  the  power  of  a  favorite  setter  to 
track  his  scent  are  well  known.  In  one  of  them  he  collected 
a  number  of  men,  and  told  them  to  walk  in  Indian  file, 
"each  man  taking  care  to  place  his  feet  in  the  footprints  of  his 
predecessor.  In  this  procession,  numbering  twelve  in  all," 
Romanes  says,  "I  took  the  lead,  while  the  gamekeeper 
brought  up  the  rear.  When  we  had  walked  two  hundred 
yards,  I  turned  to  the  right,  followed  by  five  of  the  men ;  and 


IO4  The  Animal  Mind 

at  the  point  where  I  had  turned  to  the  right,  the  seventh  man 
turned  to  the  left,  followed  by  all  the  remainder.  The  two 
parties  .  .  .  having  walked  in  opposite  directions  for  a  con- 
siderable distance,  concealed  themselves,  and  the  bitch  was 
put  upon  the  common  track  of  the  whole  party  before  the 
point  of  divergence.  Following  this  common  track  with 
rapidity,  she  at  first  overshot  the  point  of  divergence,  but 
quickly  recovering  it,  without  any  hesitation  chose  the 
track  which  turned  to  the  right."  It  had  previously  been 
ascertained  that  she  would  not  follow  the  scent  of  any 
other  man  in  the  party  save  her  master,  and  failing  him, 
the  gamekeeper.  "Yet  ...  my  footprints,"  continues  Ro- 
manes, "in  the  common  track  were  overlaid  by  eleven 
others,  and  in  the  track  to  the  right  by  five  others.  More- 
over, as  it  was  the  gamekeeper  who  brought  up  the  rear, 
and  as  in  the  absence  of  my  trail  she  would  always  follow 
his,  the  fact  of  his  scent  being,  so  to  speak,  uppermost  in 
the  series,  was  shown  in  no  way  to  disconcert  the  animal 
following  another  familiar  scent  lowermost  in  the  series" 
(367).  Such  behavior  indicates  not  only  that  the  dog  can 
experience  a  variety  of  smell  qualities,  which  is  also  the  case 
with  us  human  beings,  but  that  it  has  the  power  to  analyze 
a  fusion  of  different  odors  and  attend  exclusively  to  one 
component,  a  power  that  we  lack  almost  entirely.  When 
we  experience  two  smell  stimuli  at  the  same  time,  it  is  but 
rarely  that  we  can  detect  both  of  the  two  qualities  in  the 
mixture;  usually  one  of  them  swamps  the  other,  or  else  a 
new  odor  unlike  both  results.  But  the  dog,  and  probably 
many  other  animals,  can  analyze  a  smell  fusion  as  a  trained 
musician  analyzes  a  chord.  In  this  respect,  if  not  in  the 
variety  of  smell  qualities,  the  olfactory  sense  has  undergone 
ti  degeneration  in  us,  and  so  far  as  we  can  judge,  the  fact 
'  is  due  to  the  habit  of  relying  rather  upon  the  sense  of  sight. 


Sensory  Discrimination:  the  Chemical  Sense    105 

Even  in  the  case  of  the  monkey,  Kinnaman  reports  that  the 
animals  he  was  testing  with  regard  to  their  power  of  dis- 
criminating the  size,  shape,  and  color  of  vessels  in  one  of 
which  food  was  placed,  always  looked,  never  smelled,  for 
the  food  (221). 


CHAPTER  VI 

» 
SENSORY  DISCRIMINATION:  HEARING 

§  33.   Hearing  in  Lower  Invertebrates 

THE  sense  of  hearing,  in  all  air-dwelling  animals,  is  that 
sense  whose  adequate  stimulus  consists  in  air  vibrations; 
for  human  beings  these  vibrations  may  reach  a  frequency  of 
50,000  (single  vibrations)  in  one  second  and  still  produce  an 
auditory  sensation.  But  the  meaning  of  the  term  "  hearing  " 
for  water-dwelling  animals,  and  hence  for  most  of  the  lowest 
forms  of  animal  life,  is  more  difficult  to  determine.  Tn  the 
Protozoa  it  seems  to  have  no  meaning  at  all ;  the  reactions  of 
these  animals  to  water  vibrations  are  indistinguishable  from 
their  reactions  to  mechanical  stimulation.  But  in  some  of 
the  ccelenterates  the  possibility  of  a  specific  auditory  sensa- 
tion quality  has  been  suggested  by  the  discovery  of  a  pecul- 
iar sense  organ.  While  varying  in  its  structure  in  different 
genera  and  orders  of  ccelenterate  animals,  this  organ  con- 
sists typically  of  a  small  sac,  filled  with  fluid  and  containing 
one  or  more  mineral  bodies.  Apparently  these  latter  could 
operate  in  connection  with  a  stimulus  only  when  the  stimulus 
was  constituted  by  shaking  the  animal,  or  in  some  way  dis- 
turbing its  equilibrium.  They  might  then  serve  as  means  for 
the  reception  of  water  vibrations,  as  the  ear  serves  for  the 
reception  of  air  vibrations ;  they  might,  in  short,  be  primitive 
organs  of  hearing.  Accordingly  the  term  "  otocysts  "  was 
given  to  organs  of  this  type  wherever  they  were  found  in  the 
animal  kingdom,  and  the  mineral  bodies  in  the  otocysts  were 
called  otoliths. 

1 06 


Sensory  Discrimination  :  Hearing          107 

But  experiments  upon  coelenterates  have  entirely  failed  to 
show  that  animals  of  this  class  react  to  sounds  (in,  415,291). 
And  in  some  coelenterates,  as  well  as  in  higher  animals  having 
the  same  type  of  organ,  the  removal  of  the  so-called  otocysts 
has  been  found  to  involve  disturbance  of  the  animal's  power 
to  keep  its  balance  and  maintain  a  normal  position.  Hence 
Verworn  has  suggested  that  for  "otocyst"  and  "otolith" 
the  terms  "statocyst"  and  "statolith"  might  appropriately 
be  substituted  (415).  In  jellyfish,  indeed,  even  the  balancing 
function  of  the  statocyst  organs  appears  doubtful;  and  it  is 
possible  that  they  function  in  response  to  shaking  and  jar- 
ring (286,  291).  In  any  case,  there  is  no  evidence  whatever 
of  a  specific  auditory  sensation  in  the  consciousness,  if  such 
exists,  of  ccelenterate  animals. 

Nor  has  any  reaction  to  sound  been  demonstrated  in  either 
the  flatworms  or  the  annelid  worms;  their  sensitiveness  to 
vibrations  seems  to  be  an  affair  of  mechanical  stimulation. 
Darwin's  experiments  on  this  point  are  well  known.  The 
earthworms  which  he  observed  were  quite  insensitive  to 
musical  tones,  but  when  the  flower  pots  containing  their 
burrows  were  placed  on  a  piano,  the  worms  retreated  hastily 
as  soon  as  a  note  was  struck  (91).  Most  observers  agree  that 
mollusks  also  react  only  to  mechanical  jars  (e.g.,  101),  and 
that  the  statocyst  organs  found  in  some  mollusks  have 
no  auditory  function.  Bateson,  however,  records  that  a  cer- 
tain lamellibranch,  suspended  by  a  thread  in  a  tank,  re- 
sponded by  shutting  its  shell  when  a  sound  was  produced  by 
rubbing  a  finger  along  the  glass  side  of  the  tank  (12),  and 
Bethe  demands  to  know  of  what  possible  use  as  static  organs 
the  statocysts  in  fixed  mollusks  can  be  (27).  The  echino- 
derms  are  apparently  insensitive  to  auditory  stimuli  (350, 

365). 


io8  The  Animal  Mind 

§  34.   Hearing  in  Crustacea 

In  the  Crustacea  the  function  of  the  statocyst  organs  has 
been  the  subject  of  much  dispute.  They  are  in  this  group  of 
animals  sometimes  closed  sacs  with  statoliths,  sometimes  open 
sacs  containing  grains  of  sand.  Most  commonly  the  organs 
are  situated  in  the  basal  segment  of  the  small  antennae. 
There  is  usually  inside  the  sac  a  projection  bearing  several 
ridges  of  hairs,  graded  in  size,  which  tempt  to  the  hypothesis 
that  they  respond  to  vibrations  of  different  wave  lengths,  as 
the  fibres  of  the  basilar  membrane  of  the  human  cochlea  are 
supposed  by  the  Helmholz  theory  to  do.  Hensen,  indeed, 
placing  under  the  microscope  the  tail  of  a  small  shrimp, 
Mysis,  whose  statocyst  is  situated  in  that  region,  observed 
that  the  long  hairs  of  the  tail  vibrated  in  response  to  musical 
tones,  from  which  he  infers  that  the  statocyst  hairs  may  do  so * 
(163).  In  1899  he  was  still  inclined  to  believe  that  the  latter 
can  serve  no  other  than  an  auditory  function  (164).  Never- 
theless the  weight  of  authority  is  in  favor  of  regarding  the 
"sac"  in  Crustacea  as  a  static  rather  than  an  auditory  organ. 
The  only  evidence  of  sound  reaction  in  two  shrimp-like 
forms,  Palaemon  and  Palaemonetes,  was  a  " flight  reflex" 
given  by  some  individuals  when  sounds  were  produced  very 
near  them  in  the  water;  and  although  this  response  ceased 
when  the  statocysts  were  destroyed,  the  fact  is  of  little  sig- 
nificance, as  other  reflexes  also  were  abolished  by  the  opera- 
tion (19).  To  sounds  made  by  tapping  the  wall  of  the  aqua- 
rium Palasmonetes  reacted  by  leaping  away  from  the  wall 
nearest  to  it,  even  though  the  leap  was  made  toward  the  sound. 
When  both  statocysts  were  removed,  the  reactions  were  still 
made,  but  not  so  markedly  nor  at  so  great  a  distance  from  the 

* 

1  This  observation  is  sometimes  incorrectly  quoted  as  if  the  hairs  concerned 
were  actually  the  statocyst  hairs.  Cf.,  for  example,  Morgan,  279,  p.  266. 


Sensory  Discrimination :  Hearing         109 

sound.  A  similar  response  to  the  striking  of  a  partially 
submerged  glass  jar  was  seen  in  a  decapod,  Virbius  zos ten- 
cola,  which  has  no  statoliths  (349).  Mysis  has  been  found 
to  react  to  sounds  when  the  statocysts  are  destroyed  (27). 
The  fiddler  crab,  which  is  amphibious,  responds  in  water  to 
vibrations  by  retreating  slowly  from  the  vibrating  walls,  and 
does  the  same  when  blinded  and  deprived  of  its  statocysts, 
but  gives  no  reaction  when  the  antennae  and  antennules  are 
removed.  On  land  these  animals  do  not  respond  to  sounds, 
only  to  vibrations  produced  in  the  earth,  for  instance  by 
stamping  (349).  No  sound  reactions  have  been  found  in 
the  crayfish  (21).  In  short,  such  responses  to  vibrations  as 
occur  among  the  Crustacea  seem  affairs  rather  of  mechanical 
than  of  true  auditory  stimulation;  nevertheless  Bethe  (27) 
and  Hensen  (164)  are  both  inclined  to  believe,  as  did  Delage, 
who  first  called  attention  to  the  static  function  of  the  statocysts 
(97),  that  they  may  be  auditory  organs  also.  The  "static 
sense"  of  Crustacea  will  be  discussed  later. 

§  35.   Hearing  in  Spiders 

In  spiders  the  same  difficulty  arises,  of  deciding  whether 
the  reactions  to  sound  are  tactile  or  auditory.  There  are  no 
statocysts,  but  the  delicate  hairs  on  the  body  and  legs  of  the 
animal  have  been  held  to  be  auditory  organs.  Dahl,  a 
number  of  years  ago,  found  them  responding  to  the  tones  of  a 
violin  (86,  87),  but  this  test,  which  Hensen  applied  to  Mysis, 
is  of  very  doubtful  significance ;  as  Prentiss  suggests,  the  hairs 
on  the  back  of  the  human  hand  do  the  same  (349).  When 
various  species  of  spiders  were  tested  by  holding  tuning  forks 
near  them  or  their  webs,  only  the  web-making  species  gave  any 
response.  These  latter  would  not  react  to  ordinary  noises, 
nor  to  the  sound  of  a  small  fork,  but  to  the  humming  of  a 
large  fork  they  responded  always  by  raising  the  front  legs, 


1 10  The  Animal  Mind 

and  sometimes  by  dropping  from  their  webs  (320).  Two 
Texan  species  that  were  experimented  upon  by  placing  them 
in  a  cage  free  from  vibration  gave  no  response  whatever  to 
tuning  forks  of  various  pitches  or  to  other  sounds  (351). 
It  seems,  then,  highly  probable  that  spiders  are  sensitive 
only  to  vibrations  communicated  to  their  webs,  and  very 
likely  these  furnish  tactile  rather  than  specific  auditory  stimu- 
lation. The  observation  of  Boys  may  be  quoted:  "On 
sounding  an  A  fork,  and  lightly  touching  with  it  any  leaf  or 
other  support  of  the  web  or  any  portion  of  the  web  itself, 
I  found  that  the  spider,  if  at  the  centre  of  the  web,  rapidly 
slews  around  so  as  to  face  the  direction  of  the  fork,  feeling 
with  its  fore  feet  along  which  radial  thread  the  vibration 
travels.  Having  become  satisfied  on  this  point,  it  next  darts 
along  that  thread  till  it  reaches  either  the  fork  itself  or  a  junc- 
tion of  two  or  more  threads,  the  right  one  of  which  it  instantly 
determines  as  before.  If  the  fork  is  not  removed  when  the 
spider  has  arrived  it  seems  to  have  the  same  charm  as  any  fly, 
for  the  spider  seizes  it,  embraces  it,  and  runs  about  on  the 
legs  of  the  fork  as  often  as  it  is  made  to  sound,  never  seeming 
to  learn  by  experience  that  other  things  may  buzz  besides  its 
natural  food.  If  the  spider  is  not  at  the  centre  of  the  web 
at  the  time  that  the  fork  is  applied,  it  cannot  tell  which  way 
to  go  until  it  has  been  to  the  centre  to  ascertain  which  radial 
thread  is  vibrating."  If,  however,  it  has  followed  the  fork 
to  the  edge  of  the  web,  and  the  fork  is  then  withdrawn  and 
brought  near  again,  the  spider  reaches  out  in  its  direction. 
If  the  spider  is  at  the  centre  of  the  web  and  a  sounding  fork  is 
brought  near  without  touching  the  web,  the  spider  does  not 
reach  for  it,  but  drops  down  at  the  end  of  a  thread.  If  the 
fork  touches  the  web  again,  the  spider  climbs  the  thread  and 
finds  the  spot  very  quickly  (69). 


Sensory  Discrimination :  Hearing         1 1 1 

§  36.   Hearing  in  Insects 

The  sense  of  hearing  in  insects  also  is  problematical. 
When  the  insect  makes  a  sound  itself,  which,  as  in  the  case  of 
crickets,  is  connected  with  the  mating  process,  it  would  seem 
a  priori  highly  probable  that  it  can  hear.  Various  structures 
have  been  designated  as  auditory  organs,  the  finely  branched 
antennae  of  mosquitos  and  gnats,  on  the  same  doubtful 
evidence  that  they  have  been  found  to  vibrate  in  response  to 
musical  tones  (264) ;  and  in  the  Orthoptera  certain  very 
peculiar  structures  situated  on  the  front  legs  of  grasshoppers 
and  crickets,  and  in  the  first  segment  of  the  abdomen  in 
locusts.  These  structures  Graber  called  chordotonal  organs, 
and  he  felt  convinced  from  experimental  tests  that  they 
were  auditory.  The  cockroach,  Blatta,  while  running  about 
the  room  will  stop,  he  says,  for  an  instant  when  the  strings  of 
a  violin  are  struck.  A  blinded  specimen,  hung  by  a  thread, 
became  violently  agitated  at  a  sudden  tone  from  a  violin. 
A  water  insect,  Corixa,  although  undisturbed  by  the  water 
vibrations  produced  by  pushing  a  bone  disk  toward  it  in  the 
water,  gave  decided  reactions  when  the  disk  was  connected 
with  an  electric  bell.  Other  water  beetles  were  still  more 
sensitive.  That  they  distinguished  pitch  differences  Graber 
thought  probable  from  the  fact  that  he  observed  reactions 
of  different  degrees  of  violence  to  sounds  of  different  pitch; 
and  their  discrimination  of  intensity  changes  he  thought 
demonstrated  by  the  fact  that  if  a  continuous  tone,  sounding 
while  a  water  beetle  is  swimming  about,  be  made  suddenly 
louder,  the  speed  of  the  insect's  movements  visibly  increases. 
It  is  going  rather  far,  however,  to  pass  from  the  evidence  that 
insects  discriminate  sounds  made  by  their  own  species  from 
other  sounds  to  the  conclusion  that  "they  like  us  have  the 
capacity  to  analyze,  at  least  to  a  certain  degree,  these 


H2  The  Animal  Mind 

peculiar  clangs  or  noises,  and  to  distinguish  clearly  from  one 
another  the  partial  tones  that  compose  them"  (149). 
iTower  thought  that  he  had  observed  the  potato  foppfle, 
reacting  to  the  sound  of^a^U£ingjork^f404).  Will  noted 
responses  from  a  male  beetle  to  the  strioTulation  of  a  female 
of  its  species  enclosed  in  a  box  15  cm.  away  (439).  Radl 
has  recently  made  the  suggestion  that  the  organs  which 
Graber  called  chordotonal  organs,  and  which  contain  a  fibre 
stretched  between  two  points  of  the  integument,  represent 
a  kind  of  transition  between  " Gemeingefuhl"  and  hearing. 
In  support  he  offers  the  following  evidence:  the  fibres  re- 
semble the  tendons  in  which  some  muscles  end,  and  are  very 
likely  developed  from  tendons;  the  organs  exist  in  insects 
that  have  no  use  for  hearing,  such  as  grubs  shut  up  in  fruits; 
insects  have  not  been  shown  to  respond  to  pure  tones,  but  only 
to  noises  such  as  the  cricket's  chirping,  which  for  us  affect 
Gemeingefuhl.  Further,  there  is  no  evidence  that  hearing 
ever  guides  insects  to  each  other;  in  short,  it  is  but  a  rudi- 
mentary sense,  and  its  organs  are  those  which  serve  also  to 
register  muscular  activity.  It  is,  in  insects,  a  "  refined 
muscular  sense"  (357). 

The  auditory  sense,  if  it  exists  in  insects,  is  very  likely 
confined  to  those  which  produce  sounds,  and  its  qualities 
limited  within  the  range  of  such  sounds.  Most  species  of 
ants,  for  instance,  produce  no  sound  that  the  human  ear, 
even  with  the  aid  of  a  microphone  248),  can  detect,  although 
certain  East  Indian  species  are  reported  to  make  a  loud  hiss- 
ing noise  when  disturbed  (424),  and  some  American  species 
are  said  to  chirp  (108, 437).  Ch.  Janet  maintains  that  ants  of 
the  Myrmicidae  make  a  stridulating  noise  (190,  191).  The 
weight  of  evidence  is  also  against  the  existence  of  sound 
reactions  in  ants ;  careful  experiments  by  Fielde  and  Parker 
on  a  number  of  species  led  to  the  conclusion  that  the  only 


Sensory  Discrimination  :  Hearing          113 

vibrations  responded  to  were  those  which  were  communicated 
through  the  solid  on  which  the  ants  stood,  and  received 
through  the  legs  (125).  It  is  probable  that  the  observers 
who  have  come  to  opposite  conclusions  have  not  in  every 
case  been  careful  to  exclude  the  possibility  of  such  vibration 
of  the  substratum.  Wasmann,  for  instance,  thinks  he  has 
seen  reactions  to  sound;  he  noted  that  ants  in  an  artificial 
nest  raised  their  antennae  and  lifted  the  fore  part  of  their 
bodies  when  he  scratched  with  a  needle  on  some  sealing  wax 
with  which  the  nest  had  been  mended  (423).  He  also  quotes 
Forel's  account  (129)  of  a  species  which  makes  an  "  alarm 
signal"  by  striking  the  ground  with  its  abdomen:  this, 
remarks  Wasmann  naively,  must  be  perceived  by  the  ants, 
" otherwise  it  would  not  be  an  alarm  signal"!  (424).  If 
perceived,  it  may  of  course  be  as  a  tactile  rather  than  an 
auditory  sensation.  Weld  has  observed  reactions  to  the 
sound  of  whistles  and  tuning  forks  in  several  species  of  ants, 
and  even  concludes  that  they  perceive  the  direction  from 
which  sounds  come ;  but  since,  of  the  four  observations  upon 
which  this  latter  opinion  is  based,  two  were  cases  where  the 
ants  hurried  toward  the  sound  and  the  others  cases  where 
they  backed  away  from  it,  the  possibility  of  mere  coinci- 
dence seems  not  to  be  excluded  (433). 

As  regards  the  auditory  sense  in  bees,  there  is  again  a 
difference  of  opinion.  They  do,  of  course,  make  sounds, 
and  sounds  of  different  quality,  under  different  conditions. 
Yet  Lubbock  entirely  failed  to  get  bees  to  respond  to  any  kind 
of  sounds  artificially  produced  (248),  while  Bethe  urges  that 
the  sounds  produced  by  bees  are  involuntary,  like  the  sounds 
of  our  own  breathing  and  heart-beats,  and  that  there  is  no 
more  evidence  that  bees  can  hear  them  than  that  we  can  hear 
these  sounds  in  our  own  case  (32).  Forel  is  positive  that 
insects  in  general  cannot  hear  (130).  Von  Buttel-Reepen, 


H4  The  Animal  Mind 

on  the  other  hand,  who  knows  bees  thoroughly,  thinks  that 
the  sense  of  hearing  plays  a  considerable  part  in  their  life. 
He  believes  that  the  disturbance  produced  by  the  loss  of  a 
queen  is  communicated  to  the  whole  hive  by  the  peculiar 
wailing  noise  made  by  some  members  and  instinctively 
imitated  by  the  others,  and  that  this  disturbance  is  calmed 
by  a  similar  dissemination  of  the  " happy  humming"  pro- 
duced on  her  restoration  —  hearing  playing  a  more  important 
part  than  smell.  The  starting  of  a  swarm,  he  thinks,  is  also 
largely  a  matter  of  sound  communication.  The  process  be- 
gins by  the  coming  out  of  certain  bees  which  push  in  among 
the  bees  hanging  at  the  entrance  of  the  hive  and  stir  them  up 
to  swarming  by  making  sounds.  The  "swarm-tone"  is 
peculiar  and  often  disturbs  the  inhabitants  of  neighboring 
hives  that  are  not  ready  to  swarm.  Also,  a  swarm  can  be 
guided  to  a  new  dwelling  if  a  few  bees  are  taken  there ;  they 
call  the  others  by  loud  humming.  If  during  this  process  the 
new  hive  is  moved,  the  bees  will  go  on  for  a  few  moments  in 
the  direction  in  which  they  started,  then  slowly  turn,  guided 
by  the  tone.  A  few  may  keep  on  in  the  original  direction. 
We  may  look  with  suspicion,  however,  upon  von  Buttel- 
Reepen's  suggestion  that  these  latter,  having  passed  beyond 
hearing  of  the  call,  are  guided  by  the  recollection  of  the  tone 
they  heard  at  first !  He  refers  also  to  the  shrill  noise  made  by 
the  young  queens  ready  to  swarm,  and  to  the  peculiar  uneas- 
iness produced  when  a  strange  queen  is  being  attacked,  and 
resulting,  he  thinks,  from  her  " cries  of  pain"  (72). 

§  37.    Hearing  in  Fishes 

Throughout  the  vertebrate  animals  there  exist  structures 
bearing  analogy  to  our  own  ears,  whose  function  might  there- 
fore be  supposed  to  be  auditory.  But  in  the  lowest  verte- 
brates the  only  structures  of  the  human  ear  represented  are 


Sensory  Discrimination  :  Hearing         115 

the  semicircular  canals,  and  these  suggest  a  static  rather  than 
an  auditory  organ.  The  cyclostomes,  eel-like  and  semi- 
parasitic  forms  classed  below  the  true  fishes,  have  a  pair  of 
sacs  one  on  either  side  of  the  head,  containing  mineral  bodies, 
and  each  leading  into  one  or  two  semicircular  canals.  In 
the  true  fishes  the  sac  has  two  chambers,  marked  off  from 
each  other  by  a  constriction.  Three  semicircular  canals  open 
from  the  foremost  chamber,  two  lying  in  the  vertical  plane, 
and  one  in  the  horizontal  plane.  The  chambers  contain 
"statoliths"  and  fluid. 

That  the  semicircular  canals  in  fishes  have  a  static  l  func- 
tion has  been  shown  by  experiments  to  be  described  later.  Is 
the  fish  ear  also  an  organ  of  hearing?  Again  authorities 
differ,  and  it  is  probable  that  species  differ.  Kreidl  got  no 
response  from  goldfish  when  vibrating  rods  were  placed  either 
in  the  water  or  in  the  air  near  the  water.  Only  when  the  fish 
were  made  more  sensitive  by  strychnine  did  they  react,  and 
only  to  noise,  not  to  tone.  They  reacted  quite  as  well,  more- 
over, when  the  ears  were  removed ;  whence  it  was  concluded 
that  their  sensitiveness  to  noise  resided  in  the  skin  (227,  228). 
A  similar  negative  conclusion  regarding  auditory  sensation 
has  been  reached  by  F.  S.  Lee  (230)  and  by  O.  Korner  as  a 
result  of  experiments  on  twenty- five  species  (223).  On  the 
other  hand,  Bigelow  found  that  the  goldfish  on  which  he  ex- 
perimented were  sensitive  in  their  normal  condition,  but  in- 
sensitive when  the  auditory  nerves  were  cut,  and  thinks  that 
Kreidl's  operation  did  not  remove  the  whole  of  the  fish's  ear 
(33).  Triplett  thought  both  perch  and  goldfish  were  excited 
by  the  sound  of  whistling,  which  usually  preceded  their  being 
fed  (407).  Parker  tested  the  killifish,  a  species  of  minnow, 
using  the  sustained  slow  vibrations  (40  complete  swings  per 

1  The  word  "static"  is  here  used  to  mean  "relating  to  equilibrium"  in 
general,  not  to  static  equilibrium  as  distinguished  from  dynamic  equilibrium. 


n6  The  Animal  Mind 

second)  of  a  bass  viol  string  placed  on  one  side  of  the  aquarium 
as  a  sounding  board.  The  fish  cage  was  suspended  in  the 
aquarium  from  an  independent  support.  Normal  fish  re- 
sponded to  the  vibrations,  usually  by  movements  of  the  fin, 
96  per  cent  of  the  time.  Fish  in  which  the  nerves  to  the  ears 
had  been  cut  responded  in  18  per  cent  of  the  tests;  those  in 
which  the  skin  had  been  made  insensitive,  but  the  ears  left, 
94  per  cent.  Since  causing  the  string  to  vibrate  jarred  the 
whole  aquarium  somewhat,  these  experiments  were  checked 
by  others  where  the  stimulus  was  produced  by  placing  the 
stem  of  a  vibrating  tuning  fork  against  the  sounding  board. 
The  results  were  the  same  as  in  the  first  set  of  tests.  Parker 
concludes  that  the  ears  of  the  minnow  are  certainly  organs 
for  the  reception  of  sound ;  but  as  he  obtained  no  such  reac- 
tions from  dogfish,  he  is  inclined  to  think  that  different  species 
vary  (305,  306).  Tests  by  Zenneck  on  Leuciscus  rutilus,  L. 
dobula,  and  Alburnus  lucidus  also  led  to  the  conviction  that 
these  fish,  at  least,  could  hear.  A  bell  was  struck  by  elec- 
tricity under  water,  and  occasionally  a  piece  of  leather  was 
placed  upon  it  at  the  point  where  the  clapper  struck.  In  the 
latter  case  the  mechanical  vibrations  produced  were,  it  was 
held,  the  same  as  those  occasioned  by  the  actual  ringing  of  the 
bell,  but  the  sound  vibrations  were  destroyed.  The  fish 
reacted  by  swimming  instantly  away  from  the  neighborhood 
of  the  bell  when  it  was  rung,  but  not  when  the  leather  was 
used;  hence,  apparently,  they  reacted  to  sound  (475). 

Widely  distributed  among  fishes  is  a  curious  set  of  structures 
known  as  the  lateral-line  canals.  Along  each  side  of  the  fish, 
extending  from  head  to  tail,  there  is  a  row  of  pores  opening 
into  a  long  canal,  which  at  the  head  divides  into  three  branches, 
one  going  upward  above  the  eye,  a  second  below  the  eye,  and 
a  third  down  toward  the  lower  jaw.  The  functions  of  these 
canals  have  given  rise  to  much  discussion  among  zoologists,  an 


Sensory  Discrimination  :  Hearing          117 

exhaustive  history  of  which  will  be  found  in  Parker's  mono- 
graph entitled  "The  Function  of  the  Lateral-line  Organs 
in  Fishes."  The  problem  seems  to  have  been  solved  by 
Parker's  own  experiments.  He  first  proved  experimentally 
that  the  canals  played  no  part  in  responses  to  the  following 
stimuli :  light,  heat,  salinity  of  the  water,  food,  oxygen  dis- 
solved in  the  water,  carbon  dioxide,  foulness  of  the  water, 
hydrostatic  pressure,  steady  currents  flowing  through  the 
water,  and  sound.  When,  however,  the  water  in  the  aqua- 
rium was  made  to  vibrate  slowly,  about  six  times  per  second, 
the  fish  made  certain  characteristic  reactions,  differing 
somewhat  for  the  four  or  five  species  observed,  but  always 
failing  to  appear  when  the  lateral-line  nerve  was  cut.  Parker 
concludes  that  "the  stimulus  for  the  lateral-line  organs  (a 
water  vibration  of  low  frequency)  is  a  physical  stimulus  inter- 
mediate in  character  between  that  effective  for  the  skin  (de- 
forming pressure  of  solids,  currents,  etc.)  and  that  for  the  ear 
(vibrations  of  high  frequency),  and  indicates  that  these  organs 
hold  an  intermediate  place  between  the  two  sets  of  sense  organs 
named"  (309).  The  ear  is  thus  regarded  as  actually  derived 
from  the  lateral-line  canal,  as  this  in  turn  was  derived  from 
the  skin.  We  may  suppose  that  at  least  three  different  sensa- 
tion qualities  result  from  stimulation  of  the  skin,  the  canals, 
and  the  ear,  where  hearing  can  be  shown  to  exist. 

§  38.  Hearing  in  Amphibia 

Emergence  from  the  water,  on  the  part  of  adult  Amphibia, 
is  accompanied  by  disappearance  of  the  lateral-line  canals, 
and  consequently  of  whatever  sensations  these  mediate.  In 
the  frog,  the  ear  has  a  tympanic  membrane  lying  at  the  sur- 
face of  the  head.  A  single  bone,  the  columella,  with  one 
end  against  this  membrane,  lies  across  the  middle  ear.  The 
internal  ear  is  not  essentially  different  in  structure  from  that 


n8  The  Animal  Mind 

of  the  fish;  there  is  no  cochlea.  Yerkes  has  made  an 
interesting  study  of  the  reaction  of  frogs  to  sound.  He 
found  that  they  occasionally  ''straightened  up  and  raised  the 
head  as  if  listening"  when  other  frogs  croaked  or  made  a 
splash  by  jumping  into  the  water.  To  no  other  sound  did  he 
get  any  apparent  response,  nor  was  it  possible  to  make  frogs 
in  their  native  habitat  jump  or  show  any  uneasiness  by  pro- 
ducing any  sort  of  noise,  so  long  as  the  experimenter  remained 
invisible.  "Apparently,"  Yerkes  says,  "they  depend  almost 
entirely  upon  vision  for  the  avoidance  of  dangers."  It  is  of 
course  highly  improbable  that  an  organ  should  be  adapted 
only  to  the  reception  of  the  croaking  of  other  frogs  and  the 
splash  of  water,  and  not  to  noises  made  in  imitation  of  these ; 
and  Yerkes  suggests  that  the  frogs  may  hear  many  sounds  to 
which  they  respond  by  inhibiting  movement  as  a  measure  of 
safety.  This  view  is  confirmed  by  the  results  of  experiments 
where  the  breathing  movements  of  the  frog's  throat  were 
registered  by  means  of  a  lever  resting  against  it  and  recording 
on  smoked  paper.  Evidence  from  change  of  the  breath- 
ing rate  was  obtained  of  the  hearing  of  sounds  ranging  from 
fifty  to  one  thousand  single  vibrations  a  second  (456).  Later, 
it  was  shown  that  sounds,  although  they  did  not,  when  given 
alone,  cause  the  frogs  to  react,  modified  the  responses  to  other 
stimuli,  reinforcing  or  inhibiting  them  according  to  the  inter- 
val between  the  sound  and  the  other  stimulus.  This  effect 
was  noticed  both  when  the  frogs  were  in  the  air  and  when  they 
were  under  water.  It  was  more  marked  in  the  spring  (the 
mating  season)  than  in  the  winter.  That  it  concerned  the 
special  auditory  sense- apparatus,  and  hence  may  have  been 
accompanied  by  true  auditory  sensations,  was  shown  by  the 
fact  that  it  disappeared  when  the  auditory  nerves  were  cut. 
Sounds  ranging  from  fifty  to  ten  thousand  single  vibrations 
a  second  were  effective  (462,  464).  This,  of  course,  does  not 


Sensory  Discrimination  :  Hearing          119 

mean  that  the  frog  perceives  such  sounds  as  differing  in  pitch. 
The  absence  of  a  cochlea  throws  doubt  on  such  a  supposition ; 
the  sensation  differences  are  probably  much  cruder  than 
would  be  the  case  for  a  human  being. 

§  39.  Hearing  in  Higher  Vertebrates 

The  reptilian  ear  does  not  differ  markedly  from  that  of 
amphibians.  The  writer  knows  of  no  experiments  upon  the 
sense  of  hearing  in  reptiles. 

The  cochlea  is  supposed  to  be  the  portion  of  the  human 
ear  upon  which  the  power  to  distinguish  pitch  differences 
rests.  Yet  birds  have  no  cochlea,  though  if  we  grant  that 
animals  which  produce  sounds  are  those  which  are  able  to 
hear  them,  some  birds  at  least  must  be  capable  of  pitch 
discriminations  of  wide  range  and  great  acuteness.  The 
powers  of  imitation  so  often  evidenced  in  bird  song  are 
proof  that  this  is  the  case.1  The  sense  of  hearing,  so  long 
absent  or  problematical  in  the  ascending  scale  of  animal 
forms,  reaches  great  importance  in  the  life  of  birds  and 
mammals.  How  far  various  mammals  have  the  same 
range  of  auditory  qualities  that  a  human  being  has,  what 
their  capacity  for  pitch  discrimination  may  be,  has  been 
but  little  investigated.  Raccoons  have  been  taught  to  dis- 
criminate between  the  note  Aj  on  a  harmonica  and  the 
note  A'",  climbing  on  a  box  to  be  fed  when  the  high 
note  was  sounded  and  staying  down  when  they  heard  the 
low  one  (82).  It  is  probable  that  the  variety  of  auditory 
qualities  entering  into  the  experience  of  the  highest  verte- 
brates is  large. 

1  Interesting  evidence  of  this  power  in  a  bird  which  might  not  have  been 
supposed  to  possess  it  was  obtained  by  Conradi,  who  found  that  English 
sparrows  reared  by  canaries  acquired  recognizable  bits  of  the  canary  song 
(83). 


CHAPTER  VII 
SENSORY  DISCRIMINATION:  VISION 

§  40.  Some  Problems  connected  with  Vision 

IN  this  chapter  we  shall  consider  one  aspect  only  of  the 
reactions  of  animals  to  light  stimulation ;  namely,  the  question 
whether  such  stimulation  produces  in  the  possible  conscious- 
ness of  a  given  animal  any  sensations  qualitatively  unlike  those 
accompanying  other  forms  of  stimulation,  and  if  so,  how 
many  such  specifically  visual  sensations,  qualitatively  differ- 
ent from  each  other,  the  animal  may  be  supposed  to  be  capable 
of  receiving.  The  spatial  aspect  of  vision  will  for  the  present 
be  neglected. 

Even  with  this  restriction,  the  photic  reactions  of  animals 
present  a  series  of  problems  of  enormous  complexity.  One 
especially  difficult  question  is,  it  is  true,  postponed :  the  ques- 
tion as  to  just  what  happens  when  an  animal  seeks  or  avoids 
light.  The  so-called  orientation  of  animals,  that  is,  their 
assumption  of  a  definite  position  with  reference  to  a  force 
acting  upon  them  at  a  certain  point,  is  a  subject  more  closely 
connected  with  spatial  than  with  qualitative  discrimination; 
and  though,  as  we  shall  see,  the  seeking  or  avoiding  of 
light  by  an  animal  by  no  means  always  involves  orientation 
of  the  body,  yet  the  complex  distinctions  that  have  to  be  drawn 
in  connection  with  this  subject  will  be  more  fully  discussed 
under  the  head  of  orienting  reactions.  But  puzzles  enough  are 
left  for  the  present  chapter.  What,  for  instance,  is  the  mean- 
ing of  the  fact  that  the  rays  beyond  the  violet  end  of  the  spec- 
trum, invisible  to  us,  produce  effects  upon  certain  animals? 

120 


Sensory  Discrimination :  Vision  121 

Are  they  seen,  or  do  the  sensations  accompanying  them  rather 
resemble  those  produced  by  an  irritating  chemical?  What 
kind  of  sensation  quality  may  we  suppose  exists  in  the  con- 
sciousness of  an  animal  whose  responses  to  light  are  mediated 
by  the  skin,  not  by  the  eyes  ?  When  an  animal  discriminates 
in  its  reactions  between  rays  that  to  our  eye  differ  in  color,  is 
the  discrimination  one  of  color  qualities,  or  of  differences  in 
brightness,  such  as  the  spectrum  offers  to  a  totally  color-blind 
person  ?  And  if  a  colored  ray  does  not  produce  a  color  sen- 
sation in  the  consciousness  of  a  given  animal,  that  is,  if  the 
animal  is  color-blind,  does  it  produce  the  same  brightness 
sensation  that  it  would  produce  in  a  color-blind  human  being  ? 
These  questions  will  constantly  suggest  themselves,  but  in 
most  cases  the  evidence  will  be  insufficient  to  settle  them. 

§  41.  Vision  in  Protozoa 

Many  of  the  Protozoa,  as  we  know,  react  to  light.  Amoeba 
gives  a  negative  response  when  light  falls  upon  it  from  the 
side ;  that  is,  it  moves  away  from  the  light,  and  Jennings  con- 
jectures that  this  probably  occurs  by  the  contraction  of  the 
part  of  the  body  nearest  the  light,  which  is  what  would  happen 
if  the  light  were  a  mechanical  stimulus  (211,  p.  n).  Blue 
light  has  the  same  effect  as  white  light,  and  red  light  has  no 
effect  at  all ;  but  the  reactions  of  Amceba  to  light  of  different 
colors  differ  only  in  degree,  and  do  not  indicate  any  qualita- 
tive difference  of  accompanying  conscious  processes  (162). 
Nor,  if  the  reaction  to  light  is  really  identical  with  the  negative 
reaction  in  general,  can  we  conclude  that  any  specific  visual 
sensation  accompanies  it.  The  same  holds  true  of  the  re- 
sponses of  various  ciliate  and  flagellate  Protozoa  to  light. 
These  all,  so  far  as  observed,  take  place  by  the  ordinary  nega- 
tive or  avoiding  reaction ;  some  of  the  animals  give  it  on  pass- 
ing from  a  region  of  less  to  one  of  greater  illumination,  and 


122  The  Animal  Mind 

thus  "seek"  the  darker  regions,  while  others  give  it  when 
undergoing  a  change  in  the  reverse  direction,  and  hence  are 
"  positively  phototropic."  But  if  nothing  distinguishes  the 
negative  reaction  to  photic  stimuli  from  the  negative  reaction 
to  any  other  stimulus,  then  nothing  shows  the  existence  of  a 
sensation  quality  peculiar  to  the  effect  of  light  —  unless  a 
special  receptive  apparatus  can  be  demonstrated.  In  a  flag- 
ellate Protozoon  called  Euglena,  a  pigment  spot  exists  near 
the  anterior  end.  Now  although  pigment  apparently  is  not, 
as  Hesse  (176)  has  emphasized,  a  necessary  constituent  of 
visual  organs,  yet  its  occurrence  always  suggests  some  rela- 
tion to  light,  as  it  is  essentially  a  kind  of  matter  having  the 
property  of  absorbing  light.  Euglena  gives  the  negative  re- 
action on  entering  a  shadow.  Is  its  pigment  spot  really  an 
"eye  spot"  and  concerned  in  this  response?  Apparently 
the  reaction  occurs  before  the  pigment  spot  has  entered  the 
shadow,  and  as  soon  as  the  transparent  tip  lying  in  front  of 
the  pigment  spot  has  been  pushed  into  the  shaded  region 
(no).  It  is  uncertain,  then,  what  the  function  of  the  pigment 
spot  is.  But  in  another  organism,  which  is  structurally 
intermediate  between  the  single-celled  and  the  many-celled 
forms,  pigment  spots  do  play  a  r61e  in  light  reactions.  This 
organism  is  called  Volvox,  and  it  is  really  a  colony  of  globular 
flagellates,  each  with  its  flagellum  turned  outward,  and  each 
with  an  "eye  spot."  Very  weak  light  has  no  effect  on  the 
movements  of  Volvox;  moderate  light  causes  movement 
toward  the  source  of  light,  and  very  strong  light  causes  move- 
ment away  from  the  source  (183).  Accurate  observation  of 
these  movements  indicates  that  the  eye  spots  are  essential  to 
them;  each  individual  responds  to  a  change  of  illumination 
of  its  eye  spot  (262).  This  much  evidence,  then,  we  have  that 
if  Volvox  possesses  consciousness,  changes  of  light  intensity 
produce  in  it  a  specific  sensation. 


Sensory  Discrimination  :  Vision  123 

§  42.  Vision  in  Coslenterates 

Turning  to  the  coelenterates,  we  find  that  Hydra  shows 
no  response  to  light  other  than  a  tendency  to  come  to  rest 
in  the  more  illuminated  parts  of  the  vessel  containing  it 
(406,  444).  Very  strong  light,  however,  makes  it  wander 
about  until  it  happens  to  reach  a  more  shaded  region. 
Thus  if  the  animal  is  subjected  to  light  either  above  or 
below  a  certain  "optimum,"  it  is  restless.  A  "vague  uneasi- 
ness" is  the  kind  of  psychic  accompaniment  to  this  behavior 
most  naturally  suggested ;  repeated  strong  mechanical  stimu- 
lation will  also  make  the  animal  wander  about.  Nothing 
points  to  the  existence  of  a  visual  quality.  Blue  and  green 
light  are  more  frequented  by  Hydra  than  red  and  yellow 
light;  this  parallels  the  effect  of  colored  rays  on  Amoeba 
(444).  Widely  distributed  through  the  animal  kingdom  is  a 
kind  of  equivalence,  for  reaction  purposes,  between  blue  or 
violet  and  white  light  on  the  one  hand,  red  light  and  dark- 
ness on  the  other. 
i 

On  the  hydroid  colonies  of  Tubularia  no  change  of  light 
intensity  operated  as  a  stimulus  (319).  In  Actinians  the 
only  evidence  that  the  reactions  due  to  light  differ  from  those 
otherwise  produced  lies  in  the  greater  slowness  of  the  former. 
Many  sea-anemones  are  wholly  unaffected  by  photic  stimu- 
lation, Sagartia  lucia  and  Metridium,  for  example  (160). 
Many  others  have  been  found  to  contract  when  illuminated 
(150,  207,  291).  Eloactis  producta  expands  its  tentacles  only 
in  light  of  low  intensity,  taking  about  fifteen  minutes  to  do  so 
when  covered  with  a  hood,  and  retracting  in  five  minutes 
when  the  light  is  restored.  This  retraction  is  decidedly 
slower  than  that  produced  by  mechanical  stimulation  (160). 
That  the  responses  to  light  are  more  marked  in  animals  which 
have  been  living  in  comparative  darkness  than  in  those 


124  The  Animal  Mind 

taken  from  illuminated  spots,  has  been  shown  both  for  sea- 
anemones  and  for  Hydra  (127). 

Many  Medusae  or  jellyfish  also  react  to  light  more  slowly 
than  to  other  forms  of  stimulation.  It  is  true  that  on  Sarsia, 
a  form  tested  by  Romanes  many  years  ago,  light  seemed  to 
act  as  quickly  as  any  other  stimulus.  If  a  flash  of  light  were 
allowed  to  fall  on  the  animal  while  it  was  moving  about, 
"prolonged  swimming  movements"  ensued;  if  it  was  at  rest, 
it  gave  only  a  single  contraction  —  another  instance  of  the 
effect  of  physiological  condition  upon  reaction.  Sudden 
darkening  produced  no  reaction,  whence  Romanes  concluded 
that  "it  is  the  light  per  se  and  not  the  sudden  nature  of  the 
transition  from  darkness  to  light  which  in  the  former  experi- 
ment acted  as  the  stimulus."  There  are,  however,  as  we 
shall  see,  other  animals  in  which  an  increase  of  illumination 
brings  about  response  where  a  decrease  fails,  and  vice  versa. 
When  a  beam  of  light  was  thrown  into  a  bell- jar  containing 
many  Sarsiae  and  placed  in  a  dark  room,  "they  crowded  into 
the  path  of  the  beam  and  were  most  numerous  at  that  side 
of  the  jar  which  was  nearest  the  light."  "There  can  thus," 
concludes  Romanes,  "be  no  doubt  about  Sarsia  possessing 
a  visual  sense"  (365,  p.  41).  But  as  these  reactions  are  not 
differentiated  in  any  way,  they  cannot  be  taken  as  evidence  of 
a  specific  sense,  unless  indeed  they  depend  on  a  specialized 
sensory  structure.  This  latter  Romanes  found  to  be  the 
case ;  Sarsia  has  pigment  spots  on  the  margin  of  its  bell,  and 
its  response  to  light  ceased  when  these  were  destroyed. 
Tiaropsis,  another  jellyfish  studied  by  the  same  observer, 
gave  further  evidence  of  "a  visual  sense"  in  the  fact  that  it 
responded  to  light  more  slowly  than  to  mechanical  stimulation. 
In  Gonionemus,  both  difference  in  reaction  time  and  depend- 
ence of  response  on  a  special  organ  indicate  that  light  may 
produce  a  specific  sensation,  always  granting  the  presence  of 


Sensory  Discrimination  :  Vision  125 

consciousness.  Yerkes  found  that  this  jellyfish,  unlike  Sarsia, 
reacts  in  the  same  manner  in  passing  either  from  sunlight  to 
shadow  or  the  reverse.  In  both  cases  it  stops  swimming  and 
sinks  to  the  bottom.  A  sudden  change  of  illumination,  there- 
fore, checks  its  activity.  On  the  other  hand,  if  when  the 
light  falls  upon  it  the  animal  is  at  rest,  it  becomes  active 
again ;  but  sudden  decrease  of  illumination  has  no  effect  upon 
the  resting  animal.  The  inhibitory  effect  of  strong  light  fall- 
ing upon  the  jellyfish  while  in  motion  Yerkes  explains  as  a 
special  adaptation.  For  one  case  of  such  increase  of  illumi- 
nation occurs  when  the  animal  swims,  bell  upward,  to  the 
surface  on  being  disturbed;  the  light  of  the  surface  is  of 
course  normally  stronger  than  that  in  the  lower  regions.  The 
inhibition  of  activity  resulting  causes  the  animal,  after  turning 
over,  to  sink  slowly,  bell  downward,  with  expanded  tentacles. 
This  is  a  position  that  gives  it  a  better  chance  of  catching  food 
and  carrying  it  to  the  lips  than  is  offered  by  the  right-side-up 
posture,  where  food  would  have  to  be  carried  downward 
against  the  upward  current  occasioned  by  the  sinking  of  the 
animal.  Light  is  not  the  only  factor  in  producing  the  inver- 
sion at  the  surface,  however,  for  it  will  occur  in  darkness. 
When  swimming,  Gonionemus  moves  toward  the  light  if  the 
latter  is  fairly  intense,  but  comes  to  rest  in  the  shaded  portions 
of  the  vessel  containing  it.  The  reaction  time  to  light  is 
much  slower  than  that  to  other  stimuli,  but  the  animal  re- 
sponds most  promptly  when  certain  pigmented  bodies  at  the 
base  of  the  tentacles  are  exposed  to  the  stimulus.  If  the  mar- 
gin of  the  bell  containing  these  bodies  is  cut  off,  no  reaction 
to  light  can  be  obtained  (451,  458,  470).  A  great  variety  of 
structures  apparently  sensory  in  function  is  found  on  the  bell 
margin  of  different  genera  and  species  of  Medusae.  Some 
of  them  are  statocysts.  Others  suggest  a  visual  function, 
and  in  the  Cubomedusae  there  are  fairly  well  developed  eyes. 


126  The  Animal  Mind 

§  43.  Vision  in  Planarians 

In  planarians,  unmistakable  eyes  are  present,  yet  appar- 
ently the  reactions  to  light  are  not  wholly  dependent  upon 
them.  The  general  effect  of  photic  stimulation  on  the  plana- 
rian  is  to  stimulate  it  to  movement;  it  comes  to  rest  in  the 
shaded  portions  of  a  vessel  (9,  239,  243,  169).  To  a  certain 
extent,  light  directs  the  movement  of  the  animal  away  from  it 
(313).  But  Hesse  found  that  one  species  of  planarian  with 
much  more  highly  organized  eyes  than  another  reacted  to 
light  decidedly  less;  the  strength  of  the  light  reaction  does 
not,  he  concludes,  correspond  to  the  development  of  the  light 
perception.  The  latter  depends  on  the  number  of  sensitive 
elements  in  the  eye,  the  former  on  the  habits  of  the  animal 
and  the  feeling  tone  aroused  by  the  light  (169).  This  "  feeling 
tone"  may  apparently  be  connected  with  a  skin  sensation. 
Decapitated  and  hence  eyeless  planarians  respond  to  light, 
but  more  slowly  (243),  and  with  less  definite  reference  to  the 
direction  of  the  light  (313). 

§  44.  Vision  in  Annelids 

The  earthworm's  sensitiveness  to  light  is  also  dermal, 
although  Hesse  believes  that  he  has  found  visual  organs  in 
certain  structures  in  the  skin,  especially  at  the  upper  lip  and 
the  tail  end  (168).  However  this  may  be,  the  effectiveness  of 
light  as  a  stimulus  is  not  confined  absolutely  to  any  one  region 
of  the  body.  When  the  worms  are  in  a  normal  condition, 
attached  to  their  burrows,  the  combined  effect  of  light  and  the 
contact  stimulus  at  the  tail  produces  the  ordinary  negative 
reaction  of  withdrawal  into  the  burrow  (91,  179).  The  only 
evidence  that  light  is  accompanied  by  a  specific  conscious- 
ness is  to  be  derived  again  from  the  fact  that  the  reaction  time 
to  light  is  much  longer  than  that  to  mechanical  stimulation. 


Sensory  Discrimination:  Vision  127 

If  the  worm  is  detached  from  the  burrow,  it  will  take  a  course 
leading  it  more  or  less  obliquely  away  from  the  light ;  if  it  is 
crawling  in  passages  between  glass  plates,  which  allow  it  the 
choice  between  only  two  paths,  one  straight  toward  the  light 
and  the  other  straight  away,  it  takes  the  latter  about  95  per 
cent  of  the  time  (38 7 ).  Graber  used  his ' '  Preference  Method ' ' 
on  earthworms,  employing  a  box  with  two  compartments,  one 
illuminated  with  diffused  daylight,  the  other  dark.  At  the 
end  of  every  hour  the  number  of  worms  in  each  compartment 
was  counted.  That  in  the  darkness  was  on  the  average  5.2 
times  as  great  as  that  in  the  light.  When  ground  glass  was 
substituted  for  the  dark  screen,  making  the  compartment 
under  it  about  half  as  light  as  the  other,  the  number  in  the 
lighter  compartment  was  about  .6  the  number  in  the  darker, 
though  still  moderately  light,  portion  of  the  box,  thus  show- 
ing that  the  worms  were  sensitive  to  comparatively  small 
differences  in  intensity.  Graber  also  placed  colored  glasses 
over  the  compartments,  with  the  following  results :  the  worms 
preferred  red  to  blue  even  when  the  former  was  much  lighter 
than  the  latter,  indicating  that  the  preference  was  determined 
by  the  wave  length  and  not  by  the  brightness  of  the  light; 
they  preferred  green  to  blue  under  similar  conditions,  and  red 
to  green.  They  emphatically  preferred  white  light  from 
which  the  ultra-violet  rays  had  been  subtracted  to  ordinary 
white  light,  6.7  times  as  many  being  found  in  a  compartment 
covered  by  a  screen  impervious  only  to  ultra-violet  rays  (150). 
The  effect  of  ultra-violet  rays  on  many  animals  is  very  dele- 
terious (167).  The  avoidance  of  the  ultra-violet  rays  and 
the  seeking  of  red  by  negatively  phototropic  animals  is  almost 
universal. 

The  part  of  the  body  of  the  earthworm  affected  by  the  light 
influences  the  reaction.  Darwin  indeed  reported  that  the 
worms  withdrew  into  their  burrows  only  when  light  fell  on 


128  The  Animal  Mind 

the  head  end  (91),  but  decapitated  worms  were  found  by 
Graber  to  show  the  same  light  and  color  "preferences"  as 
normal  ones,  though  in  a  less  marked  degree  (150),  and  Yung 
obtained  evidence  that  sensitiveness  to  light  is  distributed 
over  the  body  (473).  According  to  Hesse  the  anterior  end 
of  the  worm  is  most  sensitive,  the  tail  next,  and  the  middle 
region  least  (168).  Not  only  the  region,  but  the  amount  of 
body  surface  affected,  also  makes  a  difference.  When  the 
whole  length  of  the  worm  was  illuminated,  the  percentage  of 
reactions  was  to  that  obtained  where  the  front  third  only  was 
involved  as  26  to  10.2,  while  the  relative  occurrence  of  re- 
sponses when  the  middle  third  and  the  posterior  third  alone 
were  stimulated  is  represented  by  the  figures  2.4  and  i  re- 
spectively (312).  The  effect  of  colored  rays  has  been  found 
to  be  proportionate  to  their  intensity ;  that  is,  the  green  and 
yellow  regions  of  the  spectrum  are  most  effective  (473). 

Although  the  ordinary  response  of  the  earthworm  to  light 
is  negative,  it  has  been  possible  to  determine  experimentally 
a  positive  phototropism  in  Allolobophora  fatida  for  very 
low  intensities,  and  the  emergence  of  worms  from  their 
burrows  at  nightfall  has  been  referred  to  this  tendency  to 
seek  very  faint  light  (i). 

No  parallel  in  our  own  experience  can  be  found  for  the  sen- 
sation received  by  an  eyeless  animal  from  light.  In  many 
of  the  marine  worms,  however,  well- developed  eyes  exist,  but 
not  such  as  are  capable  of  giving  clear  images.  The  function 
of  the  eyes  of  marine  worms  seems  to  be  chiefly  that  of  receiv- 
ing stimuli  from  shadows.  Many  tube-dwelling  worms  will 
withdraw  into  their  tubes  if  a  shadow  is  cast  upon  them,  and 
the  term  "skioptic"  has  been  suggested  for  this  class  of  reac- 
tions (158,  173,  373).  The  leech  Clepsine  shows  the  same 
kind  of  behavior ;  the  slightest  shadow  cast  on  the  surface  of 
the  water  in  a  dish  where  these  animals  are  resting  quietly 


Sensory  Discrimination  :  Vision  129 

will  cause  them  to  reach  up  and  sway  from  side  to  side  in 
apparent  search  for  prey  (438). 

A  curious  effect  of  colors  on  tube-dwelling  worms  has  been 
observed.  When  placed  under  either  blue  or  red  glass,  the 
sensory  activities  of  the  worms  seemed  to  be  inhibited  for 
a  time,  the  effect  being  more  striking  in  the  case  of  the  red 
glass.  When  brought  suddenly  from  under  blue  glass  into 
ordinary  white  light,  the  worms  showed  intensified  reactions ; 
while  bringing  them  from  under  red  glass  to  white  light  in- 
hibited their  reactions  for  from  two  to  five  minutes  (158). 

The  fact  that  animals  which  display  positive  phototropism 
show  also  an  "aversion"  to  red  and  a  tendency  to  seek  colors 
that  contain  the  ultra-violet  rays,  while  negatively  phototropic 
animals  avoid  light  that  has  ultra-violet  rays,  and  seek  red, 
which  lacks  these  rays,  has  pointed  to  the  probability  that 
apparent  color  discriminations  in  the  lower  forms  of  animals 
are  really  reactions  to  the  intensity  of  the  light,  and  espe- 
cially to  the  intensity  of  the  ultra-violet  rays.  This  position, 
however,  has  recently  been  questioned  by  Minkiewicz.  He 
has  succeeded  in  changing  the  reactions  of  a  Nemertean 
worm,  Linens  ruber,  to  colored  light,  while  its  response  to 
white  light  remained  unaltered.  When  placed  in  diluted  sea 
water  the  animal  would,  after  a  day,  direct  itself  toward  violet 
rays,  although  still  negative  in  response  to  white  light.  On 
the  fourth  day  the  ordinary  " chromotropism "  was  restored; 
that  is,  the  worm  sought  red  rays.  After  two  or  three  weeks 
of  life  in  the  diluted  sea  water,  on  being  restored  to  ordinary 
sea  water  the  worm's  chromotropism  was  again  inverted, 
becoming  positive  to  the  violet  rays,  while  negative  phototrop- 
ism persisted.  Moreover,  intermediate  stages  in  the  passage 
from  the  red-  to  the  violet-seeking  phase  were  observed;  a 
stage  where,  still  positive  to  red,  the  animal  ceases  to  distin- 
guish red  from  yellow,  and  others  where  it  seeks  violet,  but  has 


130  The  Animal  Mind 

become  indifferent  to  green  and  yellow.  These  stages  last  for 
several  hours,  but  corresponding  ones  have  not  been  observed 
in  the  passage  from  the  violet  phase  back  to  the  red  phase ; 
perhaps  they  occurred  too  rapidly  to  be  noted  (274). 

§  45.    Vision  in  Mollusks 

In  the  phylum  Mollusca  we  find  eyes  of  all  grades  of  de- 
velopment, from  mere  pigment  spots  in  certain  Acephala  to 
the  elaborate  eye  of  the  squid,  with  its  lens,  iris,  and  contrac- 
tile pupil.  Such  an  eye  is  fully  capable  of  forming  an  image. 
Among  the  Acephala  there  are  many  instances  of  reaction  to 
light  in  the  absence  of  all  visual  organs.  The  sensitive  parts 
are  commonly  the  siphons,  which  are  projected  from  the 
shell  to  take  in  currents  of  water  containing  nourishment,  and 
withdrawn  in  response  to  sudden  darkening  in  some  cases,  to 
sudden  illumination  in  others,  and  in  still  other  instances  to 
either  (102,  290,  373).  In  Pecten  varius,  which  has  eyes 
on  the  border  of  its  "  mantle,"  Rawitz  found  that  a  shadow 
would  cause  reaction  provided  that  it  fell  simultaneously  upon 
a  considerable  number  of  the  eyes,  from  which  he  concludes 
that  they  may  cooperate  in  a  kind  of  mosaic  vision  (360). 

In  snails,  although  the  eyes  are  undoubtedly  concerned  in 
light  reactions,  a  certain  amount  of  skin  sensitiveness  has 
been  shown.  Helix  aspersa,  a  negatively  phototropic  ani- 
mal, when  blinded,  reacted  one-half  as  many  times  to  light 
as  when  normal;  H.  nemoralis,  positively  phototropic,  only 
one-eighth  as  many  times;  from  which  the  suggestion  was 
derived  that  the  " dermal  light-sense"  may  be  more  effective 
in  negative  than  in  positive  animals  (441).  Very  interesting 
observations  on  periodic  changes  in  the  responses  of  marine 
gasteropods  to  light  have  been  made  by  Bohn  (55),  but  these 
will  be  more  fully  considered  in  a  later  chapter.  The 
cephalopods,  with  their  highly  developed  eyes,  offer  an  inter- 


Sensory  Discrimination  :  Vision  131 

esting  field  for  the  study  of  visual  reactions,  which  is  as 
yet  almost  untouched. 

§  46.    Vision  in  Echinoderms 

The  starfish  and  sea  urchin,  among  the  echinoderms, 
depend  for  their  responses  to  light  upon  pigment  or  eye  spots 
on  the  arms.  They  are  positively  phototropic,  but  lose  this 
tendency  if  the  eye  spots  are  removed ;  a  fact  which  furnishes 
some  evidence  for  the  existence  of  a  specific  visual  quality 
(365,  398).  Romanes  found  the  sensitiveness  to  light  so  great 
in  the  individuals  examined  by  him  that  they  discriminated 
between  ordinary  pine  boards  used  to  cover  the  face  of  the 
tank  containing  them,  and  the  same  boards  painted  black, 
light  being  in  both  cases  admitted  through  a  narrow  slit  (365). 

Various  sea  urchins  have  been  found  responsive  to  shadows. 
One,  Centrostephanus  longispinus,  has  not  even  the  rudi- 
ment of  an  eye.  This  animal  in  diffuse  daylight  seeks  the 
darkest  corner  and  turns  its  aboral  pole  to  the  light.  A 
sudden  shadow  falling  on  it  causes  it  to  direct  its  spines 
toward  the  shaded  side.  The  reaction  time  involved  is  de- 
cidedly longer  than  that  to  mechanical  stimulation,  and  more- 
over, although  pieces  of  the  animal  will  react  to  the  latter, 
responses  to  shadows  depend  on  keeping  the  system  of  radial 
nerves  intact.  Hence  von  Uexkiill,  who  made  the  above 
observations,  concluded  that  a  special  set  of  nerve  fibres  is 
concerned  in  photic  reactions  (410).  Dubois  had  suggested, 
from  studies  on  the  mollusk  Pholas  dactylus,  that  in  such 
cases  the  pigment  changes  which  occur,  under  the  influence  of 
light,  over  the  surface  of  the  body,  furnish  the  stimulus1  (102), 

1  The  pigment  changes,  Dubois  thinks,  cause  contraction  of  a  muscular 
layer  lying  underneath  the  pigment,  which  contraction  excites  the  nerve 
endings.  This  arrangement,  which  he  terms  a  "sysfeme  avertisseur,"  he  be- 
lieves to  be  involved  in  the  reactions  of  low  forms  of  animals  to  various 
stimuli. 


132 


The  Animal  Mind 


but  von  Uexkull  thinks  this  impossible,  as  the  light  reactions 
occur  before  the  pigment  changes  do.  This  migratory  pig- 
ment, he  believes,  acts  merely  as  a  screen;  the  source  of 
excitation  for  the  optic  fibres  may  lie  in  another  pigment 
which  he  has  extracted  and  found  very  sensitive  to  light  (410). 

§  47.    Vision  in  Crustacea 

The  spatial  aspect  of  vision  assumes  great  importance  in 
the  arthropods,  both  because  of  the  precision  of  their  orien- 
tation to  light  in 
many  cases,  and 
because  of  the 
peculiar  func- 
tions of  the  com- 
pound eye  so 
common  in  this 
group.  This  or- 
gan appears  to  be 
specially  adapted 
to  the  vision  of 
moving  objects 
(Fig.  10).  It  con- 

FiG.  10.  —  Diagrammatic  representation  of  the  compound  sists      essentially 

eye  of  a  dragon-fly.     C,  cornea;  K,  crystalline  cone;  Q£    &    number   of 
P,  pigment;  R,  nerve  rods  of  retina;  Fb,  layer  of 

fibres;  G,  layer  of  ganglion  cells;  Rf,  retinal  fibres;  simple     CVCS     SO 

Fk,  crossing  of  fibres.     After  Glaus.  Crowded  together 

that  the  common  cornea  is,  as  it  were,  faceted,  each  facet 
belonging  to  an  eye.  These  facets  are  lens  shaped,  and  back 
of  each  lies  a  refractile  crystalline  cone.  Behind  these,  in 
turn,  are  nervous  structures,  the  rods  or  retinulae,  each  sepa- 
rated from  its  neighbors  by  a  pigment  sheath.  Light  rays 
passing  through  each  corneal  facet  probably  produce  a  single 
spot  of  light  on  the  retinula,  and  the  total  image  is  thus  a 


Sensory  Discrimination  :  Vision  133 

mosaic  formed  of  these  spots.  Into  its  characteristics,  how- 
ever, we  need  not  enter.  In  the  present  chapter  we  are 
concerned  only  with  the  evidence  that  light  stimulation  in 
general,  and  light  of  different  wave  lengths  in  particular, 
produces  specific  sensations. 

That  the  visual  reactions  of  Crustacea  are  accompanied  by 
a  special  visual  sensation,  if  we  suppose  these  animals  to  be 
conscious,  is  sufficiently  evidenced  by  their  dependence  on 
the  eyes.  To  movements  and  shadows  the  responses  are  for 
the  most  part  given.  Bateson,  watching  shrimps  and  prawns, 
noted  that  they  apparently  could  not  see  their  food  when  it 
had  been  taken  from  them  and  lay  near  at  hand,  but  quickly 
raised  their  antennae  when  an  object  was  passed  between 
them  and  the  light  (i  i).  The  little  fairy  shrimp,  Branchipus, 
will  stop  swimming  as  soon  as  the  edge  of  a  shadow  falls 
upon  it.  "Skioptic"  reactions  in  the  family  of  Cirripedia, 
to  which  the  barnacles  belong,  were  noted  by  Pouchet  and 
Joubert  in  1875,  as  well  as  the  fact  that  those  individuals 
which  were  attached  to  rocks,  where  a  sudden  shadow 
might  mean  danger,  reacted,  while  those  attached  to  floating 
objects,  and  therefore  exposed  normally  to  light  fluctua- 
tions, did  not  (348).  The  problem  as  to  whether  light 
of  different  colors  produces  different  sensations  in  the 
crustacean  consciousness  was  the  subject  of  experiments 
a  number  of  years  ago,  in  which  the  Preference  Method 
was  used.  Sir  John  Lubbock  arranged  to  have  a  sunlight 
spectrum  thrown  on  a  long  trough  containing  Daphnias, 
tiny  crustaceans  belonging  to  the  lowest  sub-class,  that 
of  the  Entomostraca  (Fig.  n).  Daphnia  is  decidedly 
positive  in  its  phototropism.  At  the  end  of  ten  minutes 
glass  partitions  were  slipped  across  the  trough  at  the  approxi- 
mate dividing  lines  of  the  spectral  colors.  The  number  of 
animals  in  each  compartment  was  then  counted.  The  ex- 


134  The  Animal  Mind 

periment  was  repeatedly  performed,  and  the  greatest  num- 
ber was  always  found  in  the  yellow-green  region  (249,  250). 
Bert  obtained  similar  results  with  the  use  of  an  electric  light 
spectrum;  but  besides  throwing  all  the  colors  at  once  upon 
the  vessel,  he  allowed  each  color  to  act  separately  through  a 
narrow  opening,  and  noted  the  speed  of  the  positive  response 
produced.  That  the  "preference"  shown  for  yellow- green 
light  is  not  a  matter  of  color  vision,  but  of  response  to  the 
greater  intensity  of  the  light  in  this  region  of  the  spectrum, 
was  suggested  by  Bert  (24),  and  Merejkowsky  showed  that 

the  larvae  of  Balanus 
and  Dias  longiremis 
manifested  no  color 
preference  when  the 
colors  were  made  of 
equal  intensity  (269). 
Lubbock  attempted 

FIG.  ii.  — Daphnia.    at,  antenna;  atl,  antennule;      to    prove     the    exist- 
OC,eye.    After  Yerkes.  ence     Qf     qualitative 

as  distinguished  from  intensive  discrimination  by  various 
modifications  of  the  experiment,  but  without  entirely  con- 
clusive results  (251,  pp.  221  fL).  Finally,  Yerkes,  working 
on  Simocephalus,  a  form  closely  related  to  Daphnia,  found 
that  when  a  gaslight  spectrum  was  used,  the  animals  col- 
lected in  the  red-yellow  region,  that  of  greatest  intensity  for 
such  light ;  and  that  if  this  region  had  its  intensity  diminished 
by  a  screen  of  India  ink  or  paraffin  paper,  the  crustacean 
moved  out  of  it  (448).  In  all  probability,  then,  the  reactions 
of  these  forms  are  not  accompanied  by  qualitatively  different 
color  sensations  corresponding  to  light  of  different  wave 
lengths. 

That  Daphnia  seeks  a  region  affected  by  the  ultra-violet 
rays  of  the  spectrum  in  preference  to  darkness,  although  the 


Sensory  Discrimination  :  Vision  1 35 

two  look  alike  to  our  eyes,  was  shown  by  Lubbock  (251,  pp. 
215  ff.).  An  effect  of  physiological  condition  suggesting  the 
law  of  general  adaptation  in  human  vision  is  evidenced  by  the 
fact  that  individual  Daphnias  which  have  been  a  long  time 
in  darkness  will  respond  to  a  lower  intensity  than  those  which 
have  been  long  exposed  to  illumination  (94).  Many  curious 
results  of  physiological  condition  upon  orientation  to  light 
in  Crustacea  will  be  discussed  later. 

Experiments  on  the  reactions  of  the  crayfish,  which  is 
moderately  negative  in  its  phototropism,  to  light  coming 
through  colored  glasses  indicate  that  the  animal  seeks  red 
when  the  light  falls  vertically,  but  shows  no  marked  prefer- 
ence when  light  is  passed  horizontally  through  the  glass. 
The  tendency  to  seek  red  is  characteristic  of  negatively  photo- 
tropic  animals,  but  in  this  case  it  seemed  to  be  stronger 
even  than  the  tendency  to  seek  black.  No  definite  proof 
of  a  specific  color  reaction  is,  however,  offered  (21).  The 
positive  reactions  to  light  of  Pycnogonids,  or  sea  spiders, 
a  curious  group  of  animals  whose  classification  is  uncertain, 
have  been  found  to  depend  on  the  presence  of  a  visual  organ 

(79). 

§  48.  Vision  in  Spiders 

Spiders  do  not  have  the  compound  eye,  but  a  number  of 
ocelli,  or  simple  eyes;  the  typical  fully  developed  inverte- 
brate eye  with  cornea,  lens,  vitreous  humor,  rod  layer,  and 
pigmented  layer  in  the  retina^the  latter  lying  in  front  of  the 
nerve  fibres  supplying  the  retina,  instead  of  behind  them  as 
in  the  vertebrate  eye.  Experiments  have  been  made  on  color 
discrimination  in  spiders;  some  by  the  Preference  Method, 
where  the  spiders  showed  an  inclination  for  red  when  offered 
a  choice  of  compartments  illuminated  through  red,  green, 
blue,  and  yellow  glass  (320) ;  others  by  attempting  to  form  an 
association  between  paper  of  a  certain  color  and  the  spider's 


136  The  Animal  Mind 

nest.  This  latter,  containing  eggs,  was  surrounded  by  col- 
ored paper,  and  when  a  spider  had  become  accustomed  to 
going  in  and  out  over  the  paper,  another  color  was  substituted, 
and  a  false  nest  made  in  another  place,  surrounded  by  the 
original  strips  of  paper.  The  spider  under  these  circum- 
stances showed  some  confusion  and  tendency  to  go  to  the 
false  nest  (321).  The  experiments  with  Daphnia  have, 
however,  suggested  a  fundamental  source  of  error  in  experi- 
ments on  the  color  vision  of  animals.  A  human  being  who 
is  totally  color-blind  is  nevertheless  able  to  discriminate 
among  objects  that  to  a  normal  eye  have  different  colors, 
because  such  objects  take  on  to  the  color-blind  eye  different 
shades  of  gray.  It  is  always  possible,  then,  unless  special 
precautions  are  taken,  that  an  animal's  apparent  discrimi- 
nations of  color  may  be  really  brightness  discriminations, 
in  some  way  analogous  to  those  made  by  the  color-blind 
person.  No  such  precautions  were  taken  in  the  experiments 
just  described,  and  the  color  sense  of  spiders  remains  unproved. 
In  blind  and  blinded  myriapods,  the  family  to  which  the 
centipede  belongs,  skin  sensitiveness  to  light  is  shown  (329, 

335)- 

§  49.  Vision  in  Insects 

The  compound  eye  again  occurs  in  insects,  together  with 
ocelli  or  simple  eyes,  the  latter  usually  placed  in  the  middle 
of  the  head.  The  respective  functions  of  the  two  kinds  of 
eyes  are  not  definitely  known,  though  there  is  a  possibility 
that  the  ocelli  may  serve  for  near  vision  and  for  vision  in  faint 
light.  Plateau,  however,  finds  that  insects  with  the  com- 
pound eyes  blinded  and  the  simple  eyes  intact  are  unable  to 
see  even  in  faint  light,  and  has  but  a  poor  opinion  of  the 
usefulness  of  the  latter.  Caterpillars,  which  have  only  sim- 
ple eyes,  depend,  he  thinks,  chiefly  on  their  long  hairs  or  on 
their  feelers  to  warn  them  of  the  approach  of  obstacles  (332). 


Sensory  Discrimination  :  Vision  137 

On  the  color  sense  of  insects  there  are  first  the  old  ex- 
periments of  Graber  by  the  Preference  Method,  whose  most 
definite  result  was  to  show  that  positively  phototropic  insects 
prefer  colors  containing  the  ultra-violet  rays,  while  the  nega- 
tively phototropic  ones  prefer  red,  from  which  these  rays  are 
absent.  No  proof  that  the  discriminations  were  made  on  the 
basis  of  color  proper  rather  than  brightness  was  forthcoming 
(151).  Similar  observations  were  made  by  Lubbock  on  ants, 
which  in  their  underground  life  are  negatively  phototropic, 
the  eggs  and  larvas  apparently  needing  darkness  in  order  to 
develop,  but  on  their  foraging  expeditions  are  comparatively 
indifferent  to  light.  They  showed  a  preference  for  red  when 
tested,  and  a  tendency  to  avoid  the  ultra-violet  rays,  so  marked 
that  they  preferred  bright  daylight  from  which  these  rays 
had  been  extracted  by  chemical  screens,  to  darkness  that  con- 
tained the  ultra-violet  (248,  pp.  207  fL).  Graber  suggested 
that  the  ultra-violet  rays  produce  a  skin  sensation  in  the  ants ; 
but  Forel  agrees  with  Lubbock  that  the  effect  is  visual,  be- 
cause he  found  that  varnishing  the  eyes  made  the  ants  in- 
different to  ultra-violet  (130).  Ants  of  the  family  Lasius  seem 
to  be  normally  insensitive  to  these  rays  (134).  It  is  just  pos- 
sible, then,  that  a  visual  sensation  of  quality  wholly  foreign 
to  our  experience  may  accompany  the  action  of  ultra-violet 
rays  on  insects.  Loeb  has  noted  that  the  relative  effect  of 
violet  and  ultra-violet  vibrations,  as  compared  with  that  of  the 
rest  of  the  spectrum,  is  greater,  the  less  developed  the  visual 
organ  (233). 

Lubbock's  experiments  on  the  color  sense  of  bees  are  more 
to  the  point  than  those  on  ants,  for  they  were  made  not  by  the 
Preference  Method,  but  by  associating  a  color  with  food.  No 
precaution,  however,  was  taken  against  the  brightness  error. 
He  found  that  bees  which  had  eaten  honey  from  blue  paper 
would  pick  out  the  blue  pieces  from  a  number  of  differently 


138  The  Animal  Mind 

colored  papers,  whose  positions  were  altered  during  the  ex- 
periments (248).  Forel  got  similar  results,  and  reports  that 
a  bumble  bee  thus  trained  would  select  all  the  blue  objects 
in  the  room  for  special  examination  (130).  Lubbock's 
tests  with  wasps  gave  negative  results. 

We  have  already  noted  the  dispute  as  to  how  far  visual 
sensations  in  general  are  involved  in  the  reactions  of  bees  to 
flowers,  and  have  seen  that  Plateau  maintains  their  relative 
unimportance  in  this  connection,  as  compared  to  smell. 
Besides  the  experiments  which  we  have  quoted  on  pp.  96- 
97,  he  adduces  the  facts  that  he  could  never  persuade  insects 
to  alight  upon  artificial  flowers,  though  these  were  not  dis- 
tinguishable by  human  eyes  from  real  ones  (336-338) ;  that 
bees  show  no  preference  for  flowers  of  any  particular  color 
(339) ;  and  that  they  often  make  errors,  in  alighting  on  closed 
buds,  seed  pods,  and  wilted  flowers,  which  indicate  defective 
vision  (341).  But  Josephine  W£ry  and  others  have  noted 
that  bees  do  seek  artificial  flowers  (434).  Even  Plateau 
does  not  deny  that  an  insect  may  perceive  flowers  from  a 
distance,  "whether  because  it  sees  the  color  in  the  same  way 
that  we  do,  or  because  it  perceives  some  kind  of  contrast 
between  the  flowers  and  their  surroundings"  (339). 

Von  Buttel-Reepen  gives  one  or  two  instances  to  show 
that  the  color  perception  of  bees  is  sometimes  influential  in 
helping  them  to  recognize  their  own  hives.  He  reports  a  case 
where  a  stock  of  bees  had  been  driven  from  their  hive  and 
scattered.  The  front  of  the  hive  was  blue.  Some  of  the 
bees  tried  to  find  their  way  into  other  hives,  and  selected  for 
their  efforts  those  which  had  blue  doors.  This  authority 
believes,  moreover,  that  the  sense  of  sight  has  occasionally 
something  to  do  with  the  reception  of  bees  into  a  foreign  hive. 
"  Robber  bees,"  which  steal  honey  from  strange  hives,  when 
they  begin  their  downward  career,  approach  the  strange 


Sensory  Discrimination  :  Vision  1 39 

dwelling  with  a  peculiar  hesitating  flight;  afterwards,  says 
von  Buttel-Reepen,  they  become  "frecher."  He  declares 
that  when  attempting  to  alight  before  a  foreign  hive  they  are 
often  driven  off  by  the  rightful  occupants  before  their  odor 
can  have  been  noticed,  and  ascribes  this  reaction  to  the  sight 
of  their  hesitating  method  of  approach.  On  the  other  hand, 
when  a  broodless  stock  joins  itself  to  one  that  has  a  brood,  the 
latter  is  induced  to  receive  them  peacefully  because  of  their 
assured  manner  (72). 

The  majority  of  bee  students  incline  to  the  belief  that  bees 
are  guided  back  to  their  hives  from  long  flights  by  visual 
memory,  though  the  phenomena  are  not  easy  to  explain. 
Solitary  wasps,  it  seems  highly  probable  from  experiments, 
find  their  nests  by  sight ;  but  this  subject  will  be  more  fully 
discussed  in  Chapter  XI. 

§  50.  Vision  in  Amphioxus  and  in  Fish 

The  vertebrate  eye  differs  in  origin  and  in  structure  from 
any  form  of  invertebrate  eye,  the  most  striking  difference  in 
structure  being  perhaps  the  situation  of  the  pigmented  layer 
of  the  retina  behind  the  nerve-fibre  layer,  which  is  respon- 
sible for  the  existence  of  the  blind  spot  where  the  trunk  of 
the  optic  nerve  breaks  through  the  retinal  layer.  Another 
point  of  unlikeness  consists  in  the  fact  that  the  invertebrate 
optic  nerves  do  not  cross  on  their  way  to  the  brain,  while  in 
the  vertebrates  there  is  either  total  or  partial  crossing  of  the 
fibres.  In  both  the  vertebrate  and  the  simple  invertebrate  eye 
the  image  is  formed  by  means  of  a  lens,  although  Nagel  has 
suggested  that  the  function  of  the  lens  in  the  lower  forms  of  eye 
is  rather  to  collect  the  light  than  to  produce  an  image  (293). 

The  reactions  of  Amphioxus  to  light  offer  as  chief  evidence 
that  they  are  accompanied  by  a  specific  sensation  quality 
the  fact  that  they  may  be  fatigued  independently  of  other 


140  The  Animal  Mind 

reactions.  The  only  structures  suggesting  a  visual  function 
are  pigment  spots  on  the  back  near  the  head,  and  other  pig- 
ment cells  distributed  down  the  back.  Amphioxus  makes 
negative  reactions  to  light,  especially  when  the  light,  from 
which  heat  rays  have  been  excluded  by  passing  it  through 
water,  is  directed  at  any  point  on  the  back,  the  most  sensitive 
region  lying  just  back  of  the  eye  spot  (225,  311).  The  eye 
spot  itself,  and  the  front  end  of  the  animal,  are  insensitive. 
Fatiguing  the  light  reactions  had  no  effect  on  response  to 
other  forms  of  stimulation  (311).  Attempts  to  test  the 
4 'color  preferences"  of  Amphioxus  by  illuminating  different 
parts  of  a  trough  with  differently  colored  lights  gave  negative 
results  (225).  A  skin  sensibility  to  light  has  been  observed 
also  in  larval  lampreys,  which  will  give  negative  reactions 
even  when  the  optic  nerves  are  cut  (310).  Blind  fish  have 
been  found  to  react  to  light,  apparently  through  the  skin  (107). 
Among  the  many  animals  whose  supposed  color  prefer- 
ences Graber  tested  were  two  species  of  fish,  but  no  con- 
vincing proof  of  their  powers  of  color  discrimination  was 
obtained  (151).  Bateson  placed  food  on  differently  colored 
tiles,  and  observed  that  the  fish  picked  it  off  most  readily 
from  white  and  pale  blue,  and  least  readily  off  dark  red  and 
dark  blue;  which  establishes  little  save  that  the  bait  was 
probably  more  conspicuous  on  the  former  (12).  Professor 
Bentley  and  the  writer  succeeded  in  getting  fairly  conclusive 
evidence  that  one  fish,  of  the  common  variety  of  chub, 
Semotilus  atromaculatus,  could  associate  a  given  pigment  with 
food.  Two  dissecting  forceps  were  used,  alike  except  that 
to  the  legs  of  one  were  fastened,  with  rubber  bands,  small 
sticks  painted  red,  while  to  those  of  the  other  similar  green 
sticks  were  attached.  The  forceps  were  fastened  to  a  wooden 
bar  projecting  from  a  wooden  screen,  which  divided  the 
circular  tank  into  two  compartments,  and  hung  down  into 


Sensory  Discrimination  :  Vision  141 

the  water.  Food  was  always  placed  in  the  red  pair  of  forceps, 
which  were  made  frequently  to  change  places  with  the  green 
ones ;  and  the  fish  was  caused  to  enter  the  compartment  half 
of  the  time  on  one  side,  and  half  of  the  time  on  the  other. 
This  was  to  prevent  identification  of  the  food  fork  by  its  posi- 
tion or  the  direction  in  which  the  fish  had  to  turn.  The  animal 
quickly  learned  to  single  out  the  red  fork  as  the  one  important 
to  its  welfare,  and  in  forty  experiments,  mingled  with  others  so 
that  the  association  might  not  be  weakened,  where  there  was 
no  food  in  either  fork,  and  where  the  forceps  and  rubber  bands 
were  changed  so  that  no  odor  of  food  could  linger,  it  never 
failed  to  bite  first  at  the  red.  Moreover,  the  probability  that 
its  discrimination  was  based  upon  brightness  was  greatly 
lessened  by  using,  when  we  experimented  without  food,  a 
different  red  much  lighter  than  that  in  the  food  tests.  The 
fish  successfully  discriminated  red  from  blue  paints  in  the 
same  way,  and  it  was  afterwards  trained,  by  putting  food 
in  the  green  fork,  to  break  the  earlier  association  and  bite 
first  at  the  green  (421), 

§  51.    Vision  in  Amphibia 

The  fact  that  the  commonest  form  of  color  blindness  in 
human  beings  affects  the  qualities  red  and  green,  and  that 
these  colors  have  the  most  restricted  area  of  visibility,  might 
tempt  one  to  the  belief  that  ability  to  distinguish  red  and  green 
is  a  late  acquisition  in  the  animal  kingdom.  So  far,  com- 
parative psychology  offers  no  support  for  this  view.  The 
fish  whose  behavior  has  just  been  described  certainly  made 
some  sort  of  distinction  between  the  colors  red  and  green. 
And  the  only  evidence  of  color  vision  in  the  Amphibia  is 
evidence  that  frogs  discriminate,  in  some  fashion,  between 
red  and  white,  although  the  difference  to  the  frog  may  be  one 
of  brightness  merely.  Yerkes,  in  studying  the  frog's  power 


142  The  Animal  Mind 

to  learn  by  experience,  caused  it  to  go  through  a  simple 
labyrinth  leading  to  a  tank  of  water.  At  the  point  where  the 
first  choice  between  two  paths  occurred,  a  red  card  was 
placed  on  one  side  and  a  white  card  on  the  other.  When  the 
frog  had  learned  to  take  the  correct  path,  toward  the  white, 
the  cards  were  exchanged,  without  any  other  alteration  in  the 
conditions;  and  the  decided  confusion  of  the  animals  in- 
dicated that  they  had  discriminated  between  the  red  and 
white  cards  and  had  learned  to  react  with  reference  to  this 
discrimination  (454). 

Two  species  of  frogs  tested  by  Ellen  Torelle  showed  posi- 
tive phototropism,  associated,  as  usual,  with  a  tendency  to 
prefer  blue  to  red  light  (401).  The  frog's  phototropism, 
moreover,  persists  even  when  the  animal  is  blinded,  although 
in  the  normal  animal  the  eyes  are  involved  in  the  reaction, 
since  it  occurs  when  the  skin  is  covered  and  the  eyes  left 
intact  (224,  308).  Skin  sensitiveness  to  light  has  been 
noted  also  in  salamanders  (103).  The  nature  of  the  "dermal 
light  sensation"  remains  a  mystery.  It  can  hardly,  in  frogs, 
be  a  painful  irritation,  since  it  produces  a  positive  response ; 
and  it  is  not  due  to  heat  rays,  for  it  occurs  when  these  are 
intercepted  by  passing  the  light  through  water.  As  Parker 
says,  radiant  heat  and  light,  "  distinct  as  they  seem  to  our 
senses,  are  members  of  one  physical  series  in  that  they  are 
both  ether  vibrations,  varying  only  in  wave  length"  (308). 
While,  then,  the  nerve  endings  in  human  skin  are  sensitive 
only  to  the  slowest  of  these  vibrations,  the  heat  rays,  those  in 
the  skin  of  the  frog  may  respond  to  the  whole  series,  with 
what  accompanying  sensation  qualities  we  cannot  say. 

§  52.  Vision  in  Other  Vertebrates 

In  some  reptilian  eyes,  and  in  those  of  all  birds,  a  few 
fishes,  and  Ornithorhyncus,  there  are  attached  to  the  ends  of 


Sensory  Discrimination  :    Vision  143 

the  cones  in  the  retina  transparent,  colored  globules  like  little 
drops  of  oil.  The  significance  of  these  colored  drops  is 
wholly  unknown.  If  they  really  transmit  to  the  cones  only 
those  colors  which  they  seem  to,  then  the  color  sensations  of  the 
animals  possessing  them  must  be  wholly  different  from  ours. 

No  experimental  tests  on  color  vision  in  reptiles  have 
been  made,  so  far  as  the  writer  is  aware.  As  for  birds,  in 
the  palmy  days  of  the  doctrine  of  sexual  selection  we  should 
have  felt  quite  sure  that  the  bright  colored  plumage  of  many 
species  indicated  ability  to  distinguish  colors.  Some  experi- 
mental evidence  of  this  power  has  been  obtained.  A  chick 
that  had  learned  to  pick  out  bits  of  the  yolk  of  a  hard-boiled 
egg  from  the  white  was  given  bits  of  orange  peel,  which  he 
seized,  but  seemed  to  find  exceedingly  distasteful.  After- 
wards he  was  for  some  time  suspicious  of  the  bits  of  yolk. 
On  the  other  hand,  after  having  learned  to  avoid  bad-tasting 
black  and  yellow  caterpillars,  he  did  not  transfer  his  aversion 
to  black  and  yellow  wasps ;  probably  their  points  of  difference 
from  caterpillars  were  so  numerous  that  the  resemblance  of 
color  was  not  attended  to  (281,  pp.  40-41). 

Color  vision  in  the  English  sparrow  and  the  cowbird  has 
been  tested  by  a  method  previously  used  on  monkeys.  A 
number  of  glasses  of  like  size  and  shape  were  covered  inside 
and  out  with  differently  colored  papers,  including  red,  yellow, 
blue,  green,  dark  and  light  gray.  These  glasses  were  placed 
in  a  row  on  a  board,  and  food  was  put  always  in  the  same 
glass,  the  position  of  which,  however,  was  changed  in  the 
different  experiments.  The  sparrow  and  cowbird  learned 
to  pick  out  the  right  vessel  under  these  conditions  (345). 
Somewhat  similar  tests  were  fairly  successful  with  pigeons, 
which  were  also  experimented  on  by  Graber's  method  of 
allowing  a  choice  between  compartments  illuminated  through 
differently  colored  glass.  Although  the  pigeons  showed  no 


144  The  Animal  Mind 

tendency  to  avoid  any  particular  color,  they  indicated  a  pref- 
erence for  green  and  blue.  This  result  it  was  attempted  to 
verify  by  pneumographic  tests,  and  a  greater  quickening  of 
breathing  was  recorded  under  green  and  blue  lights  than 
under  other  light  stimuli  (371). 

Raccoons  have  been  trained  to  discriminate  cards  of  dif- 
ferent colors  and  brightnesses  in  the  following  pairs :  black- 
white,  black-yellow,  black-red,  black-blue,  black-green, 
blue-yellow,  red-green.  The  two  last-named  discriminations 
proved  decidedly  more  difficult,  and  one  of  the  four  raccoons 
tested  never  learned  to  distinguish  red  from  green  or  blue 
from  yellow  (82). 

In  none  of  the  above  described  experiments,  however,  is 
the  brightness  error  eliminated.  Kinnaman's  color  tests  on 
monkeys  did  make  an  attempt  in  this  direction.  The  monkeys 
were  tested  with  glass  tumblers  covered  with  papers  of  differ- 
ent colors,  and  when  it  had  been  shown  that  they  were  able 
to  identify  a  vessel  of  a  particular  color  as  associated  with 
food,  the  possibility  that  their  discriminations  might  have 
been  based  on  brightness  rather  than  color  was  investigated  in 
the  following  way.  First,  the  animals'  power  of  distinguishing 
different  shades  of  gray  was  tested,  and  it  was  found  that 
they  could  barely  detect  a  difference  considerably  greater 
than  that  between  the  ''brightness  values"  of  the  colors 
used ;  that  is,  the  grays  that  a  color-blind  human  being  would 
have  seen  in  place  of  the  colors.  Secondly,  this  result  was 
confirmed  by  covering  the  glasses  with  gray  papers  varying 
in  brightness  somewhat  more  than  did  the  colors  used,  and 
finding  that  the  monkeys  distinguished  these  grays  decidedly 
less  well  than  the  colors.  Thirdly,  it  was  proved  that  a 
colored  glass  could  be  picked  out  correctly  many  times  from 
among  three  others  covered  with  gray  paper  of  the  same 
brightness  as  the  color  (221). 


Sensory  Discrimination  :  Vision  145 

The  most  elaborate  and  careful  experiments  that  have  yet 
been  made  on  vision  in   the  lower  animals  are  those   of 
Yerkes  on  the  Japanese  dancing  mouse.    The  method  con- 
sisted in  teaching  the  animals  to  associate  one  of  two  differ- 
ently illuminated  compartments  with  a  disagreeable  electric 
shock.     In   the   perfected   experiments   on   brightness   dis- 
crimination,   the    illumination    of    the    compartments    was 
varied  in  intensity  by  arranging  a  light  above  each.     One 
light  could  be  kept  at  a  constant  height  and  the  other  raised 
or  lowered.    Weber's  Law  was  proved  to  hold  for  the  one 
individual  tested ;  the  ratio  of  the  difference  in  brightness  to 
the  absolute  brightness  being  about  one-tenth,  between  the 
limits  of  5  and  80  hefners  of  absolute  brightness.     For  test- 
ing color  discrimination,  after  a  series  of  experiments  with 
colored  papers,   a  somewhat  similar   apparatus   was  used, 
the  light  being  filtered  through  colored  screens  (Fig.  12). 
No  ability  to  discriminate    green   and  blue  was  displayed 
unless  the  two  were  made  very  different  in  brightness.    Light 
blue  and  orange,  green  and  red,  violet  and  red,  were  dis- 
criminated even  when  their  brightnesses  were  considerably 
varied.     Yet  the  possibility  that  these  discriminations  were 
made  on  the  basis  of  brightness  rather  than  color  differences 
is  suggested  by  an  interesting  kind  of  evidence.     After  a 
mouse  had  learned  to  choose,  for  example,  green  rather  than 
red,  it  was  offered  a  choice  between  light  and  darkness, 
and  showed  a  uniform  preference  for  the  former,  although 
untrained  mice  do  not.     This  looks  as  though  the  green  had 
been  previously  chosen  as  the  lighter  of  the  two  colors.     If 
such  were  the  case,  the  brightness  values  of  the  colors  for  the 
mouse  must  be  quite  different  from  those  which  they  have  for 
a  human  being.     In  fact,  there  are  reasons  for  thinking  that 
the  red  end  of  the  spectrum  is  much  darker  to  the  mouse's 
than  to  the  human  eye.     Even  allowing  for  the  possibility 


FlG.  13.  —  Color  discrimination  apparatus  used  by  Yerkes  on  the  dancing  mouse. 
A,  nest-box;  B,  entrance  chamber;  R,  R,  red  niters ;  G,  green  filter;  Z,,L,  incan- 
descent lamps  in  light  box ;  5,  millimeter  scale  on  light  box ;  /,  door  between 
A  and  B;  O,  O,  doors  between  alleys  and  A. 


Sensory  Discrimination :  Vision  147 

of  discriminations  based  on  brightness  differences  other  than 
those  appreciable  to  human  beings,  Yerkes  concludes  that 
the  mice  have  a  certain  degree  of  ability  to  distinguish  red, 
green,  and  violet  as  colors.  The  mouse  retina  seems  to  be 
lacking  in  cones  (469). 


CHAPTER   VIII 

SPATIALLY  DETERMINED  REACTIONS  AND  SPACE  PER- 
CEPTION 

§  53*    Classes  of  Spatially  Determined  Reactions 

MODIFICATION  of  the  behavior  of  animals  with  reference 
to  the  spatial  characteristics  of  the  forces  acting  upon  them 
appears  at  the  very  beginning  of  the  scale  of  animal  life,  and 
throughout  is  quite  as  important  as  modification  with  refer- 
ence to  the  kind  or  quality  of  such  forces.  It  assumes  a  num- 
ber of  distinct  forms.  Some  of  these  suggest  to  us,  interpret- 
ing them  as  we  must  on  the  basis  of  our  own  experience,  no 
conscious  aspect  at  all;  they  seem  rather  mechanical  effects 
upon  a  passive  organism.  In  other  cases,  it  appears  possible 
that  the  mental  process  which  we  know  as  space  perception, 
involving  the  simultaneous  awareness  of  a  number  of  sensa- 
tions consciously  referred  to  different  points  in  space,  may 
accompany  the  reaction  of  an  animal  with  reference  to  the 
spatial  relations  of  its  environment.  And  sometimes  we  can 
only  say  that  differences  in  the  space  characteristics  of  a 
stimulus  may  modify  the  accompanying  sensation  in  some 
manner  which  yet  apparently  does  not  involve  space  per- 
ception as  we  know  it. 

Our  task  in  the  following  pages  will  then  be  to  examine 
the  different  ways  in  which  animal  behavior  is  adapted  to 
the  spatial  characteristics  of  stimuli,  and  to  ask  which  of  these 
suggest  as  their  conscious  accompaniment  some  form  of  space 
perception.  A  classification  of  spatially  determined  responses 

148 


Spatially  Determined  Reactions  149 

that  is  not,  indeed,  ideally  satisfactory,  but  may  serve  our 
purpose,  divides  them  into  five  groups :  — 

1.  Reactions  adapted  to  the  position  of  a  single  stimulus 
acting  at  a  definite  point  on  the  body. 

2.  Reactions  to  a  continuous  stimulus,  which  involve  the 
assumption  of  a  certain  position  of  the  whole  body  with  refer- 
ence to  the  stimulus :  orienting  reactions. 

3.  Reactions  to  a  stimulus   that  moves,  i.e.  that  affects 
several  neighboring  points  on  the  body  successively. 

4.  Reactions  adapted  to  the  relative  position  of  several 
stimuli  acting  simultaneously. 

5.  Reactions  adapted  to  the  distance  of  an  object  from  the 
body. 

These  forms  of  behavior  will  be  successively  discussed. 

§  54.  Class  I:  Reactions  to  a  Single  Localized  Stimulus 
Responses  to  stimulation  that  are  adapted  to  the  point 
of  application  of  the  stirmqliis  are  to  be  found  among  very 
simpleanimals.  They  may  be  subdivided  into  three  groups : 
first,  cases  where  the  part  of  the  animal  that  reacts  is  the  part 
directly  affected  by  the  stimulus;  second,  cases  where  the 
whole  animal  reacts  by  a  movement  in  the  appropriate  direc- 
tion; and  third,  cases  where  a  part  of  the  body  not  directly 
affected  by  the  stimulus  moves  toward  the  point  stimulated. 

i.  Amoeba  furnishes  an  example  of  the  first  class.  Its 
negative  reaction  occurs  by  the  checking  of  protoplasmic 
flow  at  the  point  where  a  strong  mechanical  stimulus  affects 
the  body;  its  positive  reaction  by  a  flowing  forward  of  the 
protoplasm  at  the  point  where  a  weak  stimulus  acts,  and  its 
food-taking  reaction  by  an  enveloping  flow  on  both  sides  of 
the  point  stimulated.  This  would  seem  to  be  the  most 
primitive  way  of  adapting  response  to  the  location  of  a  stimu- 
lus: the  effect  is  produced  just  where  the  force  acts,  as  it 


150  The  Animal  Mind 

might  be  upon  a  piece  of  inanimate  matter.  In  no  animal 
with  a  nervous  system,  probably,  is  the  process  quite  so  simple. 
The  bell  of  the  jellyfish  contracts  at  the  point  where  a  stim- 
ulus, mechanical  or  photic,  is  applied ;  yet  although  these 
responses  are  made  when  the  nervous  system  is  thrown  out  of 
function,  they  occur  more  slowly,  and  in  the  normal  animal 
the  nervous  tissue  is  probably  involved,  while,  of  course,  a 
long  conduction  pathway  is  traversed  when,  to  use  a  familiar 
illustration,  the  baby  pulls  back  its  hand  from  the  candle 
flame. 

2.  Paramecium  and  other  infusoria,  planarians,  the 
earthworm,  and  various  other  animals  give  us  illustrations 
of  movements  of  the  entire  body  differing  according  to  the 
point  affected  by  a  single  stimulus.  If  the  front  half  of 
Paramecium  is  touched,  the  animal  gives  the  typical  avoiding 
reaction  of  darting  backward  and  turning  to  one  side;  if 
the  hinder  end  be  touched,  it  moves  forward  (211,  p.  50). 
On  the  other  hand,  it  makes  no  difference  in  its  reactions 
to  stimuli  affecting  either  side  of  the  body;  the  turning  is 
always  to  the  aboral  side  even  when  the  stimulus  comes  from 
that  direction  (211,  p.  52).  If  strong  mechanical  stimula- 
tion be  applied  to  the  head  end  of  a  planarian,  there  is 
a  response  which  seems  to  belong  under  type  (i) :  the  head 
is  turned  away  from  the  stimulus.  If  the  hinder  region  is 
touched,  strong  forward  crawling  movements  of  the  body 
are  produced.  The  positive  reaction  in  the  planarian, 
turning  the  head  toward  the  stimulus,  also  suggests  type  (i), 
but  in  reality  it  has  been  shown  by  Pearl  to  be  a  far  more 
complex  affair  than  the  mere  flow  of  protoplasm  at  the 
stimulated  point,  and  to  involve  the  contraction  of  several 
sets  of  muscles  (316).  The  earthworm  creeps  backward 
if  the  front  half  of  the  body  is  affected,  turns  away  from 
a  stimulus  applied  to  the  side  of  the  anterior  end,  and  creeps 


Spatially  Determined  Reactions  151 

forward  if  the  stimulus  affects  the  posterior  half  of  the  body 
(210).  In  general,  a  reaction  of  type  (2)  rather  than  type  (i) 
will  occur  in  proportion  to  the  degree  in  which  an  organism's 
movements  are  coordinated  and  it  tends  to  act  as  a  whole. 

3.  One  of  the  prettiest  examples  of  the  most  highly  co- 
ordinated form  of  response  to  a  single  localized  stimulus, 
namely,  movement  of  some  other  part  of  the  body  toward 
the  point  affected,  is  to  be  found  in  the  swinging  over  of  the 
jellyfish's  manubrium  toward  the  spot  on  the  bell  touched 
by  food.  "In  the  typical  feeding  reaction,"  says  Yerkes, 
"the  manubrium  bends  toward  the  food.  If  during  such 
a  movement  the  piece  of  food  be  moved  to  the  opposite 
side  of  the  bell,  the  manubrium,  too,  in  a  few  seconds  will 
bend  in  the  opposite  direction,  that  is,  again  toward  the 
food  "  (451).  The  sea  urchin  responds  to  mechanical 
stimulation  by  moving  the  spines  toward  the  place  stimu- 
lated (410).  In  the  higher  animals  this  form  of  reaction  has 
largely  superseded  other  methods  of  adapting  behavior  to 
a  stimulus  acting  at  a  definite  point.  Where  grasping 
appendages  exist,  the  obvious  device  is  to  move  them  toward 
the  point  of  stimulation  in  order  either  to  seize  or  to  remove 
the  object.  This  involves  not  merely  that  the  effects  of  the 
stimulus  shall  diffuse  so  as  to  involve  general  locomotor 
movements,  but  that  the  effect  shall  be  exerted  very  defi- 
nitely upon  a  particular  set  of  muscles  in  a  particular  way. 
The  "scratch-reflex"  of  mammals,  and  the  reaction  whereby 
a  frog  rubs  its  hind  leg  on  the  spot  of  skin  affected  by  a  drop 
of  acid,  are  further  examples. 

What  can  we  say  regarding  the  conscious  accompani- 
ment of  the  reactions  described  under  these  three  heads? 
When  a  stimulus  applied  at  point  a  brings  about  a  reaction 
different  from  that  produced  by  precisely  the  same  stimulus 
acting  on  point  b,  are  the  accompanying  sensations  different, 


152  The  Animal  Mind 

supposing  the  animal  concerned  to  be  conscious?  If  they 
are,  the  difference  must  be  what  has  been  called  a  difference 
in  local  sign.  There  is  certainly  no  evidence  that  space 
perception  is  concerned.  Space  perception  in  our  own  ex- 
perience always  involves  the  simultaneous  awareness  of  sev- 
eral stimuli.  But  where  a  single  stimulus  only  is  operative, 
the  fact  that  reaction  to  it  is  modified  by  its  location  cannot 
mean  that  the  relations  of  that  location  to  the  location  of 
other  stimuli  are  perceived.  The  truth  is  that  space  per- 
ception is  so  constant  a  factor  in  our  own  experience  that 
we  cannot  imagine  how  a  single  sensation  can  be  modified 
in  connection  with  change  of  place  of  the  stimulus,  where 
space  perception  does  not  exist.  A  touch  at  any  point 
on  the  skin  of  a  human  .being  is  referred  to  a  definite 
point  in  a  constructed  space,  tactile  and  visual;  it  is 
given  its  proper  place  in  a  complex  of  sensations.  What 
modification  of  it  would  correspond  to  its  location  if  it  stood 
alone  in  consciousness  we  cannot  now  conceive. 

§  55.  Class  II :  Orienting  Reactions;  Possible  Modes  of 
Producing  Them 

Various  forces,  such  as  gravity,  light,  electricity,  centrif- 
ugal force,  currents  of  water  and  air,  are  all  influences 
causing  certain  organisms  to  bring  their  bodies  into  a  defi- 
nite position.  Such  reactions,  involving  the  direction  of 
the  whole  body  with  reference  to  a  continuous  force  acting 
upon  it,  are  known  as  reactions  of  orientation.  There  are 
various  ways  in  which  they  might  conceivably  take  place. 

(a)  They  might  be  due  to  the  "pull"  of  a  force  upon 
the  passive  body  of  an  animal.  In  the  case  of  gravity  or 
of  a  current  of -wind  or  water,  if  one  part  of  the  body  were 
heavier  or  offered  more  surface  to  the  force,  the  position 
assumed  could  be  explained  without  supposing  any  activity 


Spatially  Determined  Reactions  153 

on  the  animal's  part.     In  such  a  case  there  would  be  no 
reason  for  thinking  of  the  reaction  as  conscious. 

(b)  The  response  might  be  due  to  the  effect  of  a  force 
acting  unevenly  upon  the  two  sides  of  the  body,  and  thereby 
unevenly  affecting  the  motor  apparatus  on  the  two  sides, 
thus  causing  the  animal  to  turn  until  the  forces  acting  upon 
symmetrical  points  were  balanced.     This,  although  involv- 
ing activity  on  the  animal's  part,  would  not,  if  the  force 
acted  directly  on  the  muscles,  suggest  any  conscious  ac- 
companiment. 

(c)  The  orientation    might    take    place    by    a   negative 
reaction  on  the  animal's  part  to  a  definite  stimulus  given 
when  the  animal  was  in  any  other  than  the  final,  oriented 
position.     If  gravity  were  the  force  in  question,  the  stimu- 
lus might  be  the  pressure  exerted  within  the  body  by  particles 
of  different  density  or  by  the  fluid  or  mineral  bodies  in  a 
statocyst  organ.     If  the  stimulus  were  light,  the  organism 
might  be  oriented  by  giving  the  negative  reaction  when  its 
head  entered  a  region  either  brighter  or  darker  than  the 
optimum  illumination.     In  such  cases,  where  the  ordinary 
negative  reaction  is  the  only  one  involved,  there  is  no  reason 
to  suppose  the  occurrence  of  any  conscious  accompaniment, 
other  than  the  possible  unpleasantness  connected  with  that 
reaction. 

(d)  •rientation  to  gravity  might  occur  through  a  special- 
ized "righting"  reaction,  given  in  response  either  to  a  stimu- 
lus within,  say,  a  statolith  organ,  or,  as  in  the  planarian,  to 
the  absence  of  accustomed  contact  stimulation  on  one  sur- 
face of  the  body.    The  reaction  in  these  cases  being  a  special- 
ized one,  it  is  possible  that  a  peculiar  sensation  quality  might 
be  involved. 

(e)  Orientation  might  take  place  through  a   movement 
occurring  when  the  position   of  several  stimuli   perceived 


154  The  Animal  Mind 

simultaneously  was  disturbed,  and  tending  to  restore  them 
to  their  original  position.  This  is  the  principle  involved, 
as  we  shall  see,  in  explaining  the  rheotropism  or  current 
orientation  of  fishes,  and  the  anemotropism,  or  orientation 
to  air  currents,  of  insects,  as  due  to  an  instinct  to  keep  the 
visual  surroundings  the  same.  And  this  form  of  orienta- 
tion alone  suggests  a  true  space  perception  as  its  conscious 
accompaniment. 

Such  being  the  conceivable  ways  in  which  orientation  may 
be  brought  about,  what  are  the  observed  facts?  They  may 
be  considered  under  the  heads  of  orientation  to  gravity,  to 
light,  and  to  other  forces. 

§  56.    Orientation  to  Gravity:   Protozoa 

To  this  form  of  reaction  the  term  "geotropism"  or  "geo- 
taxis  "  has  been  applied.  In  various  Protozoa  negative  geo- 
tropism, or  a  tendency  to  rise  against  the  pull  of  gravity,  has 
been  observed :  first  by  Schwartz  in  two  single-celled  organisms 
frequently  classified  as  plants,  Euglena  and  Chlamydomonas 
(378);  and  eight  years  later  by  Aderholcl,  who  suggested, 
without  accepting  it,  the  theory  that  the  orientation  may  be 
due  simply  to  the  greater  weight  of  one  end  of  the  organ- 
ism's body  (2).  This  view  was  maintained  by  Verworn :  the 
action  of  gravity,  he  urged,  must  be  purely  passive.  It  can- 
not operate  as  a  stimulus  to  active  response  on  the  animal's 
part,  for  a  stimulus  is  always  a  change  in  environment,  and 
gravity  is  a  constant  force  (416).  This  ignores  the  fact  that 
the  animal's  relations  to  gravity  may  change  though 
gravity  does  not.  According  to  Verworn's  theory,  the 
geotropic  orientation  of  a  single- celled  organism  takes  place 
through  a  series  of  "little  falls"  whereby  the  heavier  end  is 
directed  downward.  Massart  opposed  this  view  on  the 
basis  of  observations  which  showed  that  the  actual  move- 


Spatially  Determined  Reactions  155 

ments  of  Jthe  organisms  did  not  correspond  to  it,  but  were  the 
result  of  active  orientation.  If  response  to  gravity  is  passive, 
then  dead  animals  should  fall  through  the  water  in  the  same 
position  as  that  assumed  by  living  animals  when  oriented 
to  gravity.  Massart  experimented  with  various  Protozoa  by 
killing  them  and  studying  their  methods  of  sinking,  which 
he  found  not  always  the  same  as  the  attitudes  assumed  in 
response  to  gravity  (259).  There  is  always  the  possibility, 
however,  that  the  methods  employed  to  kill  may  change  the 
specific  gravity  of  some  part  of  the  body.  Jensen  offered 
the  theory  that  reaction  to  gravity  may  be  due  to  the  difference 
in  the  water  pressure  on  the  two  ends  of  the  animal.  He 
asserted  that  when  the  air  pressure  on  the  water  was  reduced 
by  exhausting  the  air  above,  there  was  an  increase  in  the 
geotropism,  indicating  a  relative  rather  than  an  absolute 
sensibility  to  pressure  (215),  but  Lyon  points  out  that  this 
process  may  affect  the  animals  in  various  other  ways  besides 
altering  the  air  pressure.  Increasing  the  air  pressure,  or 
protecting  the  surface  with  oil,  has  no  effect  upon  geotropism, 
Lyon  finds,  and  he  urges  that  Jensen's  theory  requires 
enormous  sensibility  to  pressure  differences  on  the  organism's 
part,  as  great  as  that  needed  by  a  human  being  to  note  the 
difference  between  the  air  pressure  on  the  head  and  that  on 
the  feet  (255).  Another  suggestion  was  offered  by  Daven- 
port, namely,  that  negatively  geotropic  organisms  swim  in 
the  direction  where  the  greatest  resistance  to  their  progress 
is  offered.  This  is  like  one  theory  put  forward  to  explain 
rheotropism,  or  the  tendency  of  animals  to  swim  against 
currents,  and  anemotropism,  or  the  "head  against  wind" 
movement  of  insects;  and  as  R£dl  (355)  first  and  Lyon  (254) 
afterward  pointed  out,  it  assumes  the  fact  to  be  explained, 
for  only  if  an  animal  actively  opposes  a  force,  will  that  force 
exert  more  pressure  at  one  point  of  its  body  than  at  another. 


156  The  Animal  Mind 

The  theory  cannot  explain  why  an  animal  at  rest  should  be 
oriented.  Another  argument  that  tells  against  it  is  offered 
by  experiments  showing  that  animals  placed  in  solutions 
of  the  same  density  as  their  own  bodies,  in  which,  therefore, 
they  have  no  weight,  still  display  negative  geotropism,  and 
that  the  direction  of  the  response  is  not  reversed  when  the 
fluid  is  made  heavier  than  the  animals  (255).  Lyon's  own 
theory,  accepted  by  Jennings,  is  that  the  stimulus  for  geo- 
tropism is  furnished  by  the  action  of  gravity  within  the  body 
of  the  organism,  upon  substances  of  different  weight  which 
exert  varying  pressures  and  take  up  different  positions  ac- 
cording to  the  position  of  the  body  (255). 

It  has  been  shown  that  the  reactions  of  Paramecium  to 
gravity  are  modified  by  a  variety  of  conditions.  Negative 
geotropism,  in  a  sense  their  normal  condition,  is  favored  by 
plentiful  food  supply  and  by  an  increase  in  temperature 
within  certain  limits;  positive  geotropism,  movement  down- 
ward, may  be  brought  about  temporarily  by  mechanical 
shock,  by  salts  and  alkalies,  by  temperature  changes  (278, 
388)  to  which,  however,  the  animals  may  adapt  themselves; 
with  less  constancy  by  increase  in  the  density  of  the  fluid 
containing  them,  and  with  lasting  effect  by  lack  of  food. 
It  has  been  suggested  that  the  downward  movement  under 
these  circumstances  is  protective,  since  it  shields  the  animals 
from  surface  agitation  of  the  water,  from  surface  ice,  and 
from  failure  of  the  surface  food  supply  (278).  We  shall 
see  that  similar  conditions  often  change  the  direction  of  an 
animal's  response  to  light. 

§57.    Orientation  to  Gravity:  Ccdenterates 
Among  the  ccelenterates,  geotropism  is  shown  by  certain 
hydroids,  whose  stems  have  a  tendency  to  curve  upward 
and  their  "  roots  "  a  tendency  to  grow  vertically  downward 


Spatially  Determined  Reactions  157 

when  the  animals  are  placed  in  a  horizontal  position  (402). 
The  sea-anemone  Cerianthus,  whose  normal  position  is  head 
upward,  will  right  itself  if  placed  in  any  other  position, 
though  the  righting  reaction  may  be  inhibited  by  contact 
stimulation  on  the  side  of  the  animal.  It  ordinarily  lives 
with  the  body  enclosed  in  a  tube,  and  when  taken  from  its 
proper  habitat  it  seems  to  " prefer"  a  position,  even  hori- 
zontal, where  the  sides  of  the  body  are  in  contact  with  a 
solid,  to  a  vertical  position  with  its  sides  uncovered  (237). 
The  righting  reaction  of  Hydra  is  not  determined  by  gravity 
at  all;  the  animal  will  take  any  position,  vertical  or  hori- 
zontal, but  "seeks"  always  to  have  its  foot  in  contact  with 
a  solid  (418).  Some  actinians  have  shown  an  interesting 
modification  of  gravity  reaction  through  what  we  may  call 
habit.  Six  specimens  of  Actinia  equina  were  selected  that 
had  been  fixed  to  the  rocks  in  an  "  upside-down "  position, 
that  is,  with  the  mouth  end  downward ;  and  six  others  that 
had  been  right  side  up.  In  the  first  experiment  all  were 
placed  upside  down;  the  tendency  to  right  themselves  was 
decidedly  stronger  in  those  which  had  been  previously  erect. 
Similarly,  when  twelve  selected  in  the  same  way  were  all 
placed  right  side  up,  the  ones  that  had  previously  been  in 
the  reversed  position  showed  a  certain  inclination  to  reassume 
it  (143).  On  the  other  hand,  the  orientation  of  the  polyp 
Corymorpha  palma  to  gravity  was  entirely  unaffected  by 
keeping  the  animal  for  a  long  time  in  a  position  where  it 
could  not  right  itself;  it  assumed  the  upright  position  as 
soon  as  it  was  set  free  (402). 

It  was  noted  in  the  chapter  on  hearing  that  the  peculiar 
organs  occurring  in  certain  Ccelenterata  and  in  many  other 
animals,  which  were  originally  called  otocysts  because  of 
their  supposed  auditory  function,  have  had  their  name 
changed  to  that  of  statocyst  since  it  has  appeared  that  they 


158  The  Animal  Mind 

subserve  chiefly  orientation  to  gravity.  In  jellyfish,  removal 
of  these  organs  does  not  seem  to  affect  the  animal's  power  of 
keeping  its  balance;  apparently  equilibrium  is  maintained 
here  by  the  simple  action  of  gravity,  for  dead  jellyfish  float 
in  the  right-side-up  position  (286,  291).  It  has  been  sug- 
gested that  the  statocyst  organs  are  for  the  reception  of 
stimuli  produced  by  shaking,  to  which  medusae  are  ap- 
parently sensitive  (291).  Negative  geotropism  exists  in 
Gqnionemus,  which  swims  to  the_  surface  of  the  water  when 
disturbed  (470).  fnTctenophors,  the  statocyst  organ,  which 
is  usually  at  one  pole  of  the  body,  has  been  found  to  function 
as  an  organ  for  the  maintenance  of  equilibrium  (415). 

§  58.  Orientation  to  Gravity:  Planarians 
A  good  example  of  a  specially  developed  reaction  having 
for  its  result  the  " righting"  of  an  animal  in  an  abnormal 
position  is  offered  by  the  behavior  of  a  planarian  that  has 
been  turned  over  so  that  its  back  rests  on  the  surface  of 
support.  The  reaction  consists  of  a  turning  of  the  body, 
beginning  with  the  head  end,  about  the  long  axis,  so  that 
a  spiral  form  is  assumed.  The  dorsal  surface  of  the  animal 
is  convex,  the  greatest  thickness  of  the  body  being  in  the 
middle  line.  When  the  planarian  lies  on  its  back,  it  thus 
naturally  tips  to  one  side,  like  a  keeled  boat  out  of  water. 
This  side,  being  brought  into  contact  with  a  solid,  gives 
a  reaction  analogous  to  the  negative  one,  that  is,  it  extends 
or  stretches.  Such  a  stretching  of  one  side  when  the  planarian 
is  right  side  up  would  of  course  produce  a  turning  in  the 
opposite  direction,  a  negative  reaction.  In  this  case,  however, 
the  opposite  side  does  not  contract  to  allow  of  turning,  but 
maintains  the  same  length.  The  necessary  result  is  that 
the  body  is  thrown  into  a  spiral:  as  soon  as  the  ventral 
surface  of  the  head  comes  into  contact  with  the  solid,  in 


Spatially  Determined  Reactions  159 

consequence  of  the  turning,  the  negative  reaction  of  that 
end  ceases.  Thus  the  righting  is  progressively  accomplished 
(316).  The  whole  response  can  hardly  be  classed  under 
the  head  of  geotropism.  Like  that  of  Hydra,  it  is  not  made 
as  the  result  of  the  pull  of  gravity,  but  is  a  reaction  to  contact 
stimulation;  the  animal  will  crawl  in  an  upside-down  posi- 
tion as  readily  as  any  other  provided  that  the  ventral  sur- 
face and  not  the  dorsal  is  in  contact  with  a  support. 

§  59.  Orientation  to  Gravity:  Annelids 
Geotropism,  in  the  marine  worm  Convoluta  roscoffensis, 
has  been  found  to  fluctuate  with  the  rise  and  fall  of  the  tides, 
even  when  the  animal  is  removed  to  an  aquarium.  In 
normal  life  the  worms  burrow  in  the  sands  at  rising  water, 
and  come  to  the  surface  when  the  tide  retreats.  Prolonged 
exposure  to  air,  or  increase  in  the  intensity  of  the  light, 
causes  them  to  move  down  the  slope  of  the  shore  to  moist 
places.  These  movements  in  the  normal  environment  are 
represented  by  upward  and  downward  movements  of  the 
animal  when  confined  in  a  glass  tube.  Keeble  and  Gamble 
thought  these  oscillations  in  geotropism  did  not  occur  in 
darkness,  and  that  the  stimulus  bringing  them  about  was 
photic.  When  the  summation  of  light  stimuli  passes  a  cer- 
tain amount,  they  maintained,  positive  geotropism  appears ; 
when  the  after  effect  of  light  stimulation  is  dissipated,  the 
negative  phase  recurs  (140).  Bohn,  however,  finds  that  the 
oscillations  do  persist  in  darkness,  and  that  their  primary 
cause  is  the  mechanical  shock  of  the  waves,  as  is  further 
indicated  by  the  observation  that  shaking  the  tube  will 
cause  the  worms  to  descend  (35).  The  geotropism  of  Con- 
voluta is  dependent  on  the  statocyst  (140). 


160  The  Animal  Mind 

§  60.   Orientation  to  Gravity:    Mollusks 

Among  Mollusks,  the  slug  has  had  its  reactions  to  gravity 
carefully  observed.  When  placed  in  a  horizontal  position 
on  an  inclined  glass  plate,  these  animals  tend  to  turn  either 
upward  or  downward,  moving  either  with  or  against  the 
force  of  gravity.  Davenport  and  Perkins  found  that  the 
same  individuals  differed  at  different  times  in  this  respect, 
and  concluded  that  the  sense  of  the  geotaxis  was  determined 
by  obscure  conditions.  They  also  found  that  an  inclination 
of  only  7.5°  on  the  part  of  the  glass  plate,  representing  only 
13°  of  the  full  force  of  gravity,  is  sufficient  to  make  the  slugs 
orient  themselves  with  reference  to  the  pull  of  the  earth, 
though  the  precision  of  such  orientation  increases  as  the 
angle  increases  (95).  Frandsen  thought  it  was  the  weight 
of  the  posterior  part  of  the  body  that  determined  whether 
the  movement  should  be  up  or  down:  that  the  natural 
tendency  of  all  was  to  go  downward,  but  that  in  some  in- 
dividuals the  posterior  part,  which  is  poorly  controlled, 
was  heavier  than  the  anterior,  and  pulled  the  animal  around 
head  upward  (135).  The  statocyst  organs  in  a  cephalopod, 
Eledone,  have  been  shown  to  function  in  maintaining 
equilibrium  (137). 

§  61.   Orientation  to  Gravity:   Echinoderms 

Very  interesting  righting  reactions,  in  the  starfish  and  sea 
urchin,  are  described  by  Romanes.  The  starfish  rights 
itself  by  twisting  around  the  tips  of  two  or  three  of  its  rays 
until  the  suckers  in  the  ventral  side  have  a  firm  hold  of  the 
supporting  surface,  and  then  continuing  the  twisting,  always 
in  the  same  direction  on  the  different  rays,  until  the  whole 
body  is  turned.  The  sea  urchin,  "a  rigid,  non-muscular  and 
globular  mass,"  with  relatively  feeble  suckers,  has  a  much 


Spatially  Determined  Reactions  161 

harder  time  of  it,  and  does  not  succeed  in  pulling  itself  over 
unless  it  is  perfectly  fresh  and  vigorous.  It  occasionally 
rests  for  some  time  when  it  has  reached  a  position  of  stability 
halfway  over,  before  continuing  the  process  (365). 

Lyon  has  observed  marked  negative  geotropism  in  the 
larvae  of  the  sea  urchin.  He  was  unable  to  test  Davenport's 
theory  of  the  nature  of  the  geotropic  response  by  putting  the 
animals  in  a  solution  of  the  same  density  as  their  own  bodies, 
for  the  reason  that  such  a  fluid  was  too  dense  and  sticky 
(being  made  of  gum  arabic  and  sea  water)  for  them  to  swim 
in.  That  the  response  was  merely  a  passive  one  he 
thinks  improbable,  because  the  larvae  from  eggs  that  have 
been  rapidly  rotated,  or  "centrifuged,"  as  it  is  called,  have 
all  the  pigment  on  one  side  of  their  bodies  and  may  therefore 
be  supposed  to  have  their  ordinary  balance  disturbed;  yet 
they  rise  to  the  surface  just  like  the  rest  (256). 

§  62.  Orientation  to  Gravity:  Crustacea 
That  the  statocyst  organs  in  Crustacea  are  probably  con- 
nected with  equilibrium  rather  than  with  hearing  we  have 
already  seen.  Delage  in  1887  found  that  Mysis,  Palaemon, 
and  other  forms  displayed  serious  disturbance  of  equilibrium 
when  both  eyes  and  statocysts  were  destroyed,  showing  that 
the  e^es_also  play  a  part  in  the  maintaining  of  balance  (97). 
The  eyes  have  been  found  to  cooperate  with  the  statocysts 
in  the  fiddler  crab,  Gelasimus,  and  also  in  another  decapod, 
Platyonichus  (78).  Neither  of  these  has  statoliths.  Penaus 
membraneus,  on  the  other  hand,  was  found  to  be  permanently 
disoriented  by  destruction  of  the  statocysts  or  even  removal 
of  the  statoliths,  while  blinding  produced  no  great  dis- 
turbance, probably  because  of  the  animal's  nocturnal  habits 
(19,  138).  Young  crayfish  with  the  statocysts  destroyed 
will  swirn  upside  down  as  readily  as  right  side  up  (71). 


1 62  The  Animal  Mind 

But  the  prettiest  evidence  for  the  static  function  of  the 
statocysts  was  obtained  when  powdered  iron  was  substituted 
for  the  mineral  bodies  in  the  open  statocysts  of  Palaemon. 
It  was  found  that  when  a  magnet  was  brought  near,  the  animal 
would  respond  by  taking  up  a  position  corresponding  to  the 
resultant  of  the  pull  of  the  magnet  and  that  of  gravity  (226). 

Specific  righting  reactions  occur  in  many  Crustacea, 
though  in  some  cases  these  seem  to  be  merely  the  incidental 
effects  of  their  ordinary  locomotion.  Branchipus,  the  fairy 
shrimp,  normally  swims  upside  down;  if  turned  right  side 
up  when  moving  along  the  bottom  of  the  vessel,  it  continues 
to  move  in  this  position  without  showing  any  disturbance 
until  it  happens  to  rise  a  little  from  the  bottom,  when  ap-^ 
parently  the  weight  of  the  body  pulls  it  around  into  the 
usual  upside-down  position.  The  crayfish  has  two  methods 
of  righting  itself:  a  quick  "flop"  executed  with  the  tail, 
and  a  slow  and  laborious  raising  of  itself  on  one  side  and 
tipping  over  (96). 

Many  Crustacea  show  marked  responses  to  gravity: 
for  example,  Parker  found  decided  negative  geotropism  in 
the  females  of  the  marine  copepods  whose  depth  migrations 
he  studied.  It  seems  to  be  needed  to  counteract  the  tendency 
of  the  animals  to  fall  to  the  bottom  by  their  own  weight  (304). 
In  certain  copepods,  light  was  observed  to  change  the  sense 
of  the  response  to  gravity,  not  by  taking  its  place  as  a  directive 
stimulus,  but  apparently  by  producing  some  physiological 
change  in  the  animals.  Their  normal  geotropism  was 
positive,  that  is,  they  had  a  tendency  to  move  downwards. 
In  darkness,  however,  their  geotropism  became  negative. 
They  were  also  negatively  phototropic  to  strong  light.  If, 
when  in  the  negatively  geotropic  phase,  they  were  illuminated 
from  below  by  intense  light,  from  which  they  would  ordinarily 
have  moved  away,  the  change  from  negative  to  positive 


Spatially  Determined  Reactions  163 

geotropism  induced  by  the  light  was  of  sufficient  influence 
to  make  them  move  downward  toward  it  (113).  Other 
facts  regarding  the  relation  of  geotropism  and  phototropism 
are  mentioned  on  pp.  182  ff. 

§  63.  Orientation  to  Gravity:  Spiders  and  Insects 
|  Spiders  and  insects  have  no  statolith  organs.  Bethe 
^thinks  that  equilibrium  is  maintained  in  their  case  as  a 
natural  result  of  the  position  of  the  centre  of  gravity  and  the 
distribution  of  air  in  the  body.  He  supports  this  view  by 
experiments  in  which  dead  insects,  allowed  to  fall  through 
the  air,  assume  the  normal  position,  and  is  inclined  to  think 
that  all  animals  without  special  static  organs  maintain  their 
balance  in  this  way  (27).  Negative  geotropism  in  certain 
insects,  as  evidenced  by  a  tendency  to  creep  from  horizontal 
planes  up  vertical  ones,  was  observed  by  Loeb  (234).  In 
light  the  eyes  of  insects  have  probably  much  to  do  with 
maintaining  equilibrium.  Certain  aquatic  insects,  in  ex- 
periments where  the  light  was  made  to  strike  them  only 
from  below,  as  soon  as  they  left  the  support  on  which  they 
were  resting,  turned  themselves  upside  down  (355). 

§  64.  Orientation  to  Gravity:  Vertebrates 
It  has  long  been  known  that  in  vertebrates  the  static 
function  resides  in  the  ear,  and  especially  in  the  semicir- 
cular canals  (e.g.,  70,  85,  128,  147).  Various  experimenters 
have  noted  that  operations  on  the  ears  of  fishes  disturb  the 
equilibrium  of  these  animals.  Sewall,  indeed,  found  that 
section  of  the  semicircular  canals  in  the  shark  had  no  effect 
on  its  balancing  powers,  although  operations  on  the  vesti- 
bule and  ampullae  did  disturb  movement  (380) ;  and 
Steiner  got  no  effect  on  equilibrium  from  removing  the 
contents  of  the  labyrinth  (391).  Errors  in  method  and 


164  The  Animal  Mind 

observation  probably  influenced  these  results.  Loeb  found 
that  severing  the  auditory  nerve  or  removing  the  statoliths 
from  the  dogfish  caused  the  fish  to  incline  toward  the  operated 
side  and  to  roll  the  eyes  in  that  direction  (238).  Total 
extirpation  of  one  labyrinth  in  the  perch  was  observed  by 
Bethe  to  make  the  fish  curve  toward  the  affected  side.  The 
fish  Scardinius  showed  a  tendency  to  curve  toward  the 
opposite  side  (27).  Lee's  experiments  on  the  dogfish  showed 
a  very  definite  relation  between  the  position  of  the  canal 
operated  upon  and  rolling  movements  of  the  fish.  Cutting 
the  front  canals  caused  the  fish  to  dive  forward,  cutting  the 
rear  canals  made  it  dive  backward,  and  cutting  the  canal 
on  either  side  made  it  roll  over  toward  that  side.  A  natural 
explanation  of  this  behavior  is  to  suppose  that  the  absence 
of  stimulus  from  the  cut  canal  produces  the  same  effect  that 
rolling  the  fish  in  the  opposite  direction,  and  thus  diminish- 
ing the  pressure  of  the  fluid  in  the  canal,  would  produce. 
The  fish  " feels  as  if"  it  were  being  rolled  over,  and  makes 
movements  to  regain  its  equilibrium.  When  the  nerves 
supplying  the  ears  on  both  sides  were  cut,  the  fish  became 
perfectly  indifferent  to  its  position  and  would  float  upside 
down  without  any  effort  to  right  itself.  The  vestibule  and 
otoliths  of  the  fish  ear  are  thought  by  Lee  to  be  concerned 
with  statical  equilibrium;  that  is,  with  the  maintenance  of 
position  while  the  fish  is  at  rest,  while  the  canals  are  concerned 
with  balance  during  motion  (dynamic  equilibrium)  (230). 
It  may  be  added  that  experiments  on  the  sea  horse  indicate 
that  destruction  of  the  labyrinths  in  this  animal  has  no  effect 
on  equilibrium:  the  upright  attitude  is  due  to  the  position 
of  the  air  bladder  and  is  assumed  even  by  dead  animals  (139). 
That  vision  may  materially  aid  in  maintaining  equilibrium 
in  vertebrates  is  indicated  by  evidence  from  various  sources, 
among  others,  the  observation  of  Bigelow  that  goldfish  in 


Spatially  Determined  Reactions  165 

which  the  nerves  supplying  both  ears  had  been  cut  recovered 
after  two  or  three  weeks  and  could  swim  quite  normally 
except  when  they  were  placed  in  a  large  body  of  water  and 
made  to  swim  rapidly,  when  they  showed  no  power  of  pre- 
serving their  balance  (33).  Their  successful  performance 
of  slower  movements  was  very  likely  due  to  the  use  of 
sight. 

Sensory  impulses  from  the  body  muscles  themselves  un- 
doubtedly cooperate  with  those  from  the  semicircular  canals 
in  the  maintenance  of  balance.  They  are  evidently  involved 
in  the  peculiar  withdrawing  movements  by  which  land  ani- 
mals, even  puppies,  kittens,  and  young  rats  whose  eyes  have 
not  opened,  save  themselves  from  falling  when  they  reach 
the  edge  of  the  object  on  which  they  have  been  crawling 
(271,  384).  Water-dwelling  animals,  accustomed  to  plunge 
off  solid  supports,  lack  this  protective  instinct;  Yerkes 
showed  that  among  several  species  of  tortoises,  some  land- 
dwelling,  some  amphibious,  and  some  aquatic,  the  first  men- 
tioned were  much  more  reluctant  than  the  second  to  crawl 
off  the  edge  of  a  board,  and  the  second  more  reluctant  than 
the  third  (459). 

§  65.  The  Psychic  Aspect  of  Orientation  to  Gravity 
Glancing  back  over  these  examples  of  the  responses  made 
by  animals  to  gravity,  we  note  that  while  in  some  cases  the 
earth's  attraction  appears  to  act  mechanically  upon  the 
animal,  causing  the  body  passively  to  assume  a  certain 
position,  the  common  method  of  bringing  about  orientation 
seems  to  be  that  some  structure  in  the  body,  placed  in  an 
abnormal  position,  presents  a  stimulus  which  brings  about 
a  compensatory  movement.  This  structure  may  be  heavier 
particles  of  the  body  substance,  as  probably  is  the  case  in 
Paramecium ;  it  may  be  a  statolith,  or  the  fluid  in  the  laby- 


1 66  The  Animal  Mind 

rinth;  it  may  be  the  eyes.  In  any  case,  what  shall  we  say 
about  the  sensation  quality  involved  ?  Perhaps  the  reactions 
produced  are  wholly  reflex.  Perhaps  the  statolith  or  the 
canal  fluid  produces  a  specific  sensation  quality.  Or  perhaps, 
as  Verworn  thinks,  the  sensation  quality  is  merely  that  of 
pressure  (415).  Whatever  its  nature,  spatial  perception, 
the  perception  of  the  spatial  relations  between  several  stimuli 
simultaneously  apprehended,  plays  no  part  in  the  orientation 
of  animals  to  gravity. 

§  66.  Orientation  to  Light:  Photopathy  and  Phototaxis 
One  of  the  first  facts  that  confronts  the  student  of  the 
ways  and  means  by  which  animals  become  oriented  to  visual 
stimuli  is  the  distinction  which  Loeb  drew  between  what  he 
called  heliotropism  and  sensibility  to  difference  (Unterschieds- 
empfindlichkeit)  (239) ;  and  to  indicate  which  the  terms 
"  phototaxis  "  and  "  photopathy  "  have  also  been  applied. 
The  phenomena  are  as  follows.  Strasburger,  working  on 
the  swarm  spores  of  certain  plants,  thought  he  had  evidence 
that  their  reactions  to  light  evinced  not  so  much  a  tendency 
to  seek  a  certain  intensity  of  illumination,  as  a  susceptibility 
to  the  direction  from  which  the  light  came.  He  placed  over 
the  vessel  containing  them  an  India  ink  screen,  thicker  at 
one  end  so  as  to  cause  gradations  in  the  intensity  of  the  light 
reaching  the  vessel.  When  the  light  fell  perpendicularly 
through  this  screen,  the  distribution  of  the  swarm  spores 
through  the  vessel  was  nearly  uniform ;  that  is,  the  differences 
of  intensity  had  no  effect.  When  the  screen  was  removed, 
and  the  light  fell  at  an  angle,  the  spores  immediately  oriented 
themselves  to  its  direction,  and  preserved  this  orientation 
even  when  the  screen  was  replaced  (392).  The  word 
phototaxis,  instead  of  being  used  to  designate  any  reaction 
to  light,  has  been  narrowed  to  designate  the  tendency  to 


Spatially  Determined  Reactions  167 

orient  with  reference  to  the  direction  rather  than  to  the 
intensity  of  the  light. 

Now  on  the  other  hand  there  are  some  cases  where  animals 
apparently  seek  or  avoid  light  without  being  oriented  at  all ; 
that  is,  without  having  their  bodies  placed  in  a  definite  position 
during  the  movement.  Planar ians  are  an  example  of  this. 
Increased  intensity  of  light  stimulates  them  to  activity ;  they 
crawl  about  until  they  reach  a  shaded  portion,  where  they  come 
to  rest.  Their  movements  are  not  directed  straight  away  from 
the  light ;  in  other  words,  it  does  not  negatively  orient  them, 
but  it  excites  them  and  the  shadow  brings  them  to  rest  (239). 
To  cases  such  as  this,  where  a  certain  intensity  of  light 
stimulates  activity  while  a  different  intensity  may  inhibit  it,  but 
where  no  orientation  of  the  body  with  reference  to  the  direction 
of  the  rays  occurs,  the  term  "photopathy"  may  conveniently 
be  applied.  Bohn  suggests  that  the  tendency  of  certain  ani- 
mals to  come  to  rest  in  shaded  portions  may  really  be  an 
expression  of  fatigue  produced  by  the  action  of  the  light  (55). 

A  second  problem  arises  in  connection  with  the  mechanism 
of  phototaxis.  How  does  light  orient  an  animal?  Does 
it  exert  an  effect  upon  the  muscles  or  locomotor  organs  of  the 
body  that  is  equivalent  to  pulling  the  animal  around  into  the 
required  position?  Or  does  the  organism  become  oriented 
because  in  a  series  of  movements  those  which  would  bring  it 
out  of  the  oriented  position  are  corrected  by  negative  reactions  ? 
Again,  if  the  effect  of  light  upon  the  body  is  direct,  producing 
orientation  by  bringing  the  animal  at  once  into  line  with  the 
light  rays,  is  this  effect  produced  by  the  direction  of  the  light 
rays  as  they  pass  through  the  body,  or  by  the  fact  that  in  any 
other  than  the  oriented  position  two  symmetrical  points  on 
opposite  sides  of  the  body  are  illuminated  with  unequal 
intensity,  a  theory  of  phototaxis  which  would  bring  it  into 
nearer  relation  with  photopathy? 


1 68  The  Animal  Mind 

§  67.   Instances  of  Photopathy  and  Phototaxis 

The  phenomena  of  orientation  to  light  in  different  groups 
of  animals  suggest  now  one,  now  another  of  these  questions. 
In  Protozoa,  although  attempts  have  been  made  to  show  that 
orientation  is  produced  by  the  direct  effects  of  light  on  sym- 
metrical points,  according  to  the  observations  of  Jennings 
(206)  and  Mast  (261),  it  seems  to  be  due  to  negative  reactions 
given  when  the  organism,  in  its  ordinary  swimming  move- 
ments, either  passes  into  a  region  of  greater  or  less  illumina- 
tion, or  swings  its  anterior  end  "  to  ward  or  away  from  the 
source  of  light,  so  that  it  is  shaded  at  one  moment  and  strongly 
lighted  at  the  next."  That  is,  the  reactions  are  caused,  not 
by  the  direction  of  the  light  rays  as  such,  but  by  differences 
in  the  intensity  of  illumination.  Strasburger's  results,  in 
which  the  swarm  spores  moved  toward  the  light  into  a  region 
of  less  intense  illumination,  Jennings  holds  were  due  to  the 
fact  that  "turning  the  sensitive  anterior  end  away  from  the 
source  of  the  light"  would  diminish  the  effective  illumination 
of  the  animal  more  than  passing  into  the  slightly  less  illu- 
minated region.  That  is,  the  two  ways  of  changing  the  inten- 
sity of  the  stimulus,  moving  forward  into  a  darker  region, 
and  turning  the  head  end  away  from  the  light,  are  here 
opposed :  the  latter  effect  is  stronger  than  the  former,  hence 
the  organisms  do  not  turn  the  head  end  from  the  light, 
or  rather  they  make  the  negative  reaction  when  it  is  so 
turned,  and  do  move  toward  the  shaded  region.  "If  the 
difference  in  intensity  of  light  in  different  parts  of  the  drop 
were  increased  till  the  change  in  illumination  due  to  pro- 
gression is  greater  than  the  change  due  to  swinging  the 
anterior  end  away  from  the  source  of  light,  then  the  positive 
organisms  would  gather  in  the  more  illuminated  regions" 
(211,  p.  148). 


Spatially  Determined  Reactions  169 

In  Volvox,  also,  orientation  is  held  by  Oltmanns  (298)  and 
Mast  (262)  to  be  an  affair  of  intensity  differences  rather  than 
of  light  direction.  The  reaction  of  a  Volvox  colony,  which  in 
moderate  light  is  positively  phototropic,  occurs  in  consequence 
of  a  response  by  each  individual  in  the  colony  given  when, 
as  the  colony  rotates,  that  individual  passes  from  a  higher 
to  a  lower  intensity  of  light. 

In  Hydra,  the  effect  of  light  is  photopathic  rather  than 
phototactic.  We  have  seen  that  these  animals,  when  sub- 
jected to  light  either  above  or  below  a  certain  " optimum" 
intensity,  wander  about  until  they  reach  a  region  of  the  right 
degree  of  illumination ;  their  movements  manifest  no  definite 
orientation  (444).  One  sea-anemone,  Actinia  cereus,  ob- 
served by  Bohn,  does  show  an  oriented  response  to  light. 
Weak  light  causes  expansion  of  its  tentacles  perpendicularly 
to  the  light  rays.  If  the  light  is  increased,  the  tentacles  "tend 
to  orient  themselves  in  the  direction  of  the  rays,  and  finally 
converge  in  a  bundle  parallel  to  that  direction,"  a  response 
which  has  the  effect  of  protecting  them  from  the  intense 
light  (62). 

The  medusa  Gonionemus  offers  an  instance  of  opposition 
between  photopathy  and  phototaxis,  the  former  being  nega- 
tive, the  latter  positive,  in  daylight.  That  is,  it  moves  toward 
the  light  when  swimming,  but  being  less  active  in  darkness 
than  in  light,  it  comes  to  rest,  and  hence  tends  to  collect, 
in  darkened  regions.  Intense  light  gives  a  negative  photo- 
taxis.  Sudden  decrease  and  sudden  increase  of  light  intensity 
have  alike  the  effect  of  temporarily  inhibiting  activity.  On 
swimming  either  from  shadow  into  sunlight,  or  from  sunlight 
into  shadow,  the  medusa  stops,  turns  over,  and  sinks  to  the 
bottom.  But  when  this  effect  has  been  produced  by  entering 
shadow,  the  animal,  on  again  becoming  active,  may  move 
in  any  direction ;  when  it  has  been  produced  by  entering  sun- 


170  The  Animal  Mind 

light,  the  medusa  on  beginning  to  move  again  "usually  turns 
in  such  a  way  as  to  move  back  into  the  shaded  region."  This 
effect  Yerkes,  who  first  observed  it,  thinks  due  to  the  contrac- 
tion of  the  bell  on  the  more  illuminated  side ;  that  is,  it  is  a 
definite  reflex  to  a  localized  stimulus.  Orientation  results 
from  the  fact  that  the  greater  intensity  of  stimulus  on  one  side 
of  the  bell  produces  contraction  at  that  point  (451, 470,  468). 

We  have  already  seen  that  planarians  offer  an  illustration 
of  photopathy.  Light  is  not,  however,  wholly  without  effect 
on  the  direction  of  the  animal's  movements.  It  has  been 
found  that  when  planarians  are  placed  with  the  head  toward 
the  source  of  light,  they  have  a  distinct  tendency  to  turn  out 
of  the  path,  while  if  their  heads  are  directed  straight  away 
from  the  light,  their  tendency  is  to  keep  in  the  path  (313). 
It  is  probable  that  when  the  animal  turns  its  head  toward  the 
light,  its  movement  is  checked  by  a  negative  reaction.  An 
attempt  has  been  made  to  show  that  photopathy,  rather  than 
response  to  the  direction  of  the  light  rays  as  such,  governs  the 
responses  of  the  land  planarian  Bipalium  kewense  to  photic 
stimuli.  The  apparatus  was  arranged  so  that  a  shadow  was 
thrown  from  above  upon  an  area  of  light  coming  horizontally 
from  one  side.  Although  the  animal  is  negatively  phototropic 
to  a  marked  degree,  it  would  crawl  toward  the  source  of  light 
in  order  to  get  into  the  shadow  (81).  The  explanation  for 
this  may  very  likely  correspond  to  that  offered  by  Jennings  for 
the  reverse  behavior  of  Strasburger's  swarm  spores.  That 
is,  the  planarian  might  have  obtained  a  greater  diminution 
of  light  intensity  by  moving  into  the  shadow  than  by  turning 
aside  from  the  path  of  the  rays.  The  one  possibility  excluded 
is  that  the  negative  reaction  of  planarians  is  a  response  to  the 
direction  of  the  light  rays  as  such. 

The  same  evidence  for  photopathy,  as  distinct  from  photo- 
taxis  produced  by  direction  of  light,  has  been  obtained  for  the 


Spatially  Determined  Reactions  171 

earthworm  Allolobophora  fatida  (81).  Yet  the  movements 
of  earthworms  are  oriented  by  light ;  as  we  have  seen,  they 
tend  to  move  away  from  a  source  of  light.  This  orientation 
Holmes  believes  to  take  place  by  the  checking  of  random 
movements  of  the  head  in  the  direction  of  the  light.  In  the 
crawling  movements  stimulated  when  light  is  thrown  upon 
the  worm,  the  head  is  turned  from  side  to  side.  If  it  happens 
to  be  turned  toward  the  light,  it  is  withdrawn.  Holmes  ex- 
plains the  observation  of  Parker  and  Arkin  that  the  head  of 
the  worm  is  much  more  apt  to  turn  from  the  light  than  toward 
it  (312),  by  saying  that  account  was  probably  taken  here  only 
of  the  first  decided  turn  made.  He  himself  experimented  by 
lowering  a  worm,  crawling  on  a  wet  board,  while  its  body  was 
in  a  straight  line  and  contracted,  into  a  beam  of  light  at  right 
angles  to  the  body,  and  noting  the  first  movement  of  the 
head.  This  was  found  to  be  twenty-seven  times  away  from 
the  light  and  twenty-three  times  toward  the  light.  A  similar 
method  of  orientation  by  "trial  and  error''  was  observed  in 
the  leech  and  in  fly  larvae  by  Holmes  (185). 

E.  H.  Harper,  on  the  other  hand,  working  on  the  earth- 
worm PericluBta  bermudensis,  declares  that  if  the  light  is 
strong  enough  there  are  no  random  movements  of  the  head  at 
all,  but  that  the  first  movement  is  a  direct  reflex  away  from 
the  light.  When  the  light  is  only  moderate,  the  appearance 
of  random  movements  is  due  to  the  fact  that  the  worm  is  less 
sensitive  in  a  contracted  than  in  an  expanded  state.  Loco- 
motion consists  in  a  series  of  contractions  and  expansions, 
and  "as  each  extension  begins  in  a  state  of  lower  sensibility, 
the  anterior  end  may  be  projected  toward  the  light,  only  to 
be  checked  when  its  increase  of  sensibility  with  extension 
makes  the  stimulus  appreciated"  (161).  A  similar  sugges- 
tion that  orientation  may  occur  either  by  a  definite  reflex  or 
as  the  outcome  of  random  movements,  according  to  the  ani- 


172  The  Animal  Mind 

mal's  physiological  condition,  is  to  be  found  as  early  as  the 
work  of  Pouchet  on  fly  larvae.  He  noted  that  the  courses 
taken  by  the  larvae  away  from  the  light  were  either  straight, 
"or  they  present  to  right  and  left  indentations  due  to  the 
wavering  movements  which  the  animal  makes  ...  in  a 
certain  number  of  cases,  as  if  to  take  at  each  instant  a  new 
direction."  These  individual  differences  might  have  been 
accounted  for,  says  Pouchet,  by  differing  degrees  of  hunger 
in  the  larvae  (347). 

Phototaxis  in  certain  tube-dwelling  marine  worms  was  ob- 
served by  Loeb.  Spirographis  spallanzanii  gradually  curves 
its  tube  until  its  oral  end  faces  the  direction  from  which  the 
rays  of  light  come ;  and  another  marine  worm,  whose  tube  is 
absolutely  stiff,  adapts  itself  to  a  change  in  the  direction  of 
the  rays  by  curving  the  newly -formed  portions  of  the  tube  as 
it  constructs  them  (236). 

Attempts  to  show  the  independence  of  photopathy  and  pho- 
totaxis  by  causing  a  positively  phototactic  animal  to  move 
toward  the  source  of  light  even  when,  by  an  arrangement  of 
screens  overhead,  such  movement  brings  it  into  a  region  of 
dimmer  illumination,  have  been  made  with  apparent  success 
on  the  crustacean  Daphnia  (93).  That  no  increase  in  the 
intensity  of  the  light  will  reverse  Daphnia's  positive  phototaxis 
is  also  evidence  that  photopathy,  the  seeking  of  an  optimum 
intensity,  is  absent  in  these  Crustacea  (457).  Simocephalus, 
being  made  to  collect  in  the  brighter  regions  of  a  trough  and 
showing  no  orientation  to  light  rays  entering  the  trough  at 
right  angles,  seemed  to  display  photopathy  independent  of 
phototaxis  (448).  It  is  very  difficult,  however,  to  be  sure  in 
such  experiments  that  the  direction  of  the  light  rays  and  the 
intensity  of  the  illumination  are  really  independently  varied, 
for  the  diffusion  of  light  by  floating  particles  and  its  reflection 
from  the  sides  of  the  trough  offer  disturbing  factors.  The 


Spatially  Determined  Reactions  173 

amphipod  Talorchestia  longicornis,  which  moves  toward  the 
light  but  comes  to  rest  in  shaded  portions,  seems  to  combine 
positive  phototaxis  with  negative  photopathy  (181).  Loeb's 
observations  on  the  larvae  of  the  arachnid  Limulus,  the  horse- 
shoe crab,  and  upon  insect  larvae,  may  also  be  mentioned 
here.  When  strongly  negative,  the  former  moved  away  in 
the  line  of  rays  of  sunlight  falling  obliquely  from  a  window 
upon  the  vessel  containing  them ;  the  shadow  of  the  window 
bar  lay  across  the  vessel,  and  the  animals  continued  to  move 
through  it  in  the  same  direction,  although,  on  passing  out  from 
it,  they  went  into  a  more  brightly  lighted  region  (239).  A 
similar  illustration  of  phototaxis  without  photopathy  was 
found  in  the  caterpillars  of  the  Porthesia  moth,  which  give  a 
positive  response,  and  in  fly  larvae,  which  are  negative  (235). 

§  68.  Direction  and  Intensity  Theories  of  Phototaxis 
The  problem  as  to  whether  orientation  to  light  is  brought 
about  by  the  influence  of  the  direction  of  light  rays  as  such,  or 
by  the  fact  that  light  falling  upon  an  oriented  organism  from 
a  given  direction  affects  symmetrical  points  with  different 
degrees  of  intensity,  is  one  requiring  much  nicety  of  discrimi- 
nation between  concepts.  Loeb,  in  his  earliest  discussion  of 
the  subject,  expresses  himself  positively  in  favor  of  the  former 
hypothesis.  "The  orientation  of  animals  to  a  source  of  light 
is,  like  that  of  plants,  conditioned  by  the  direction  in  which  the 
light  rays  traverse  the  animal  tissue,  and  not  by  the  difference 
in  the  light  intensity  on  the  different  sides  of  the  animal" 
(233).  To  this  Bohn  urges  as  a  "fundamental  objection" 
that  "the  'luminous  rays'  which  strike  a  living  body  have, 
save  in  wholly  exceptional  cases,  various  directions,  being 
reflected,  diffused  and  refracted  by  neighboring  bodies"  (55). 
Certainly  if  definite  orientation  to  light  occurred  only  when  an 
animal's  body  was  traversed  by  rays  in  one  predominant  direc- 


174  The  Animal  Mind 

tion,  it  would  be  of  little  practical  service.  The  other  view, 
that  the  important  factor  is  the  difference  in  the  intensity  of 
stimulation  of  opposite  points  on  the  unoriented  animal's 
body,  is  that  held  by  Verworn  (417).  Holmes  points  out  that 
no  crucial  test  experiment  of  the  two  hypotheses  has  ever 
been  made.  Such  an  experiment  would  require  that  a  semi- 
transparent  animal  should  have  two  symmetrical  points  on  its 
body,  a  and  b,  stimulated  with  exactly  equal  intensity,  each 
by  a  ray  of  light  coming  from  a  different  direction.  Under 
such  circumstances,  according  to  the  theory  of  Verworn,  the 
animal  ought  to  move  straight  forward  (181).  An  attempt 
to  get  evidence  was  made  by  Davenport  and  Cannon  in  a 
study  of  Daphnia.  They  proposed  the  following  question: 
Do  positively  phototactic  animals  move  more  rapidly  toward 
their  optimum  intensity  than  toward  an  intensity  below  the 
optimum?  If  orientation  is  determined,  as  the  Verworn 
theory  supposes,  by  the  relative  intensity  of  light  on  different 
points  of  the  organism,  then  the  absolute  intensity  of  the 
light  ought  not  to  affect  it.  If,  on  the  other  hand,  the  direc- 
tion of  the  rays  orients  the  animal,  then  precision  of  orienta- 
tion should  increase  as  the  absolute  intensity  approaches  the 
optimum.  Daphnia  was  found  to  move  somewhat  less 
rapidly  toward  the  light  when  the  intensity  of  the  latter  was 
reduced ;  this  fact  was  held  to  be  due  to  diminished  precision 
of  orientation  and  hence  to  tell  for  the  theory  of  Loeb1  (93). 

§  69.    The  Eyes  in  Phototaxis 

The  directive  theory  of  phototaxis  is  of  little  significance 
in  connection  with  the  reactions  to  light  of  organisms  whose 
bodies  are  opaque  and  which  have  eyes.  For  the  eyes  seem 
to  be  fundamentally  concerned  in  orientation  to  light.  That 

1  A  discussion  of  the  intensity, and  direc^on  theories  will  be  found  in  Holt 
and  Lee's  article  on  "The  Theory  of  Phototactic  Response"  (187). 


Spatially  Determined  Reactions  175 

this  is  the  case  in  Daphnia  was  shown  by  Radl,  who  placed 
the  animal  under  a  microscope  in  such  a  way  that  only  the 
eyes  could  be  moved.  When  the  light  coming  from  below 
was  diminished,  the  eyes  rolled  upward;  when  the  light 
coming  from  above  was  diminished,  they  rolled  downward. 
The  precise  positive  phototaxis  of  Daphnia,  Ra"dl  thinks,  is 
primarily  an  eye  movement,  the  body  being  turned  to  follow 
the  eyes  (354).  Indeed,  Radl  is  of  the  opinion  that  in  all 
animals  having  eyes,  the  essential  feature  of  phototropism  is 
eye-orientation,  wholly  analogous  to  fixation  in  the  case  of 
human  vision  (356).  In  amphipods,  blackening  of  one  eye 
of  a  positively  phototropic  animal  caused  a  turning  toward  the 
blackened  side,  as  if  the  animal  were  trying  to  restore  the 
missing  illumination;  similar  experiments  upon  negative 
animals  produced  turning  toward  the  other  side  (181).  Like 
phenomena  have  been  observed  in  other  Crustacea,  in 
mollusks,  annelids,  and  insects.  Bonn,  like  Radl,  is  in- 
clined to  explain  the  light  tropisms  of  animals  with  eyes  as 
entirely  due  to  an  effect,  either  tonic  or  inhibitory,  according 
as  the  animal  is  positive  or  negative,  of  light  acting  through 
the  eyes  upon  the  muscles  of  the  same  side  of  the  body.  If 
one  eye  received  more  light  than  the  other,  a  positive  animal 
would  turn  toward  the  darker  side  because  the  muscles  on 
the  side  toward  the  light  would  be  more  strongly  stimulated. 
A  negative  animal  would  turn  toward  the  light  because  of 
inhibition  of  muscular  activity  on  that  side.  Orientation 
may  then  be  effected  in  a  normal  animal  when  the  eyes 
equally  illuminated  (55). 


:   01 

ion  \ 

are  ] 


§  70.   Influences  Affecting  the  Sense  of  Light  Orientations 
In  no  class  of  animal  responses  to  stimulation  is  the  effect 
more  dependent  upon  the  cooperation  of  a  number  of  condi- 
tions than  in  those  involving  orientation  to  light.     Many  in- 


176  The  Animal  Mind 

fluences  have  been  found  to  reverse  the  sense  of  light  reactions, 
transforming  negatively  phototropic  into  positively  photo- 
tropic  animals,  and  vice  versa.  That  such  reversal  should 
occur  in  response  to  increase  or  decrease  of  the  intensity  of 
the  light  is  what  one  would  naturally  expect ;  if  a  certain  in- 
tensity of  illumination  is  favorable  to  the  life  processes  of  an 
animal,  it  would  seem  appropriate  for  it  to  seek  light  of  that 
intensity  but  avoid  light  of  greater  intensity.  Many  animals, 
like  Gonionemus,  are  positive  to  light  of  moderate  intensity 
and  negative  to  strong  light  (451).  The  females  of  the  crusta- 
cean Labidocera  migrate  to  the  surface  of  the  water  at  night- 
fall because,  like  the  earthworm,  they  react  positively  to  faint 
light ;  and  move  downward  at  sunrise  because  they  are  nega- 
tive in  their  response  to  intenser  light  (304).  On  the  other 
hand,  Holmes  observed  that  Orchestia  agilis,  an  amphipod 
crustacean,  would,  if  brought  from  strong  to  weaker  light,  be- 
come negative  for  a  short  time ;  the  meaning  of  such  a  change 
it  is  difficult  to  conjecture  (181).  Sudden  reduction  of  light 
causes  a  temporary  negative  phase  also  in  Convoluta  roscof- 
fensis  (140). 

Prolonged  action  of^Jight__ma,y  alter  phototropism :  the 
"depth  migrations,"  that  is,  the  periodical  movements  toward 
and  away  from  the  surface  of  the  water,  in  the  free- swimming 
larvae  of  the  barnacle,  Balanus,  are  due  apparently  to  the  fact 
that  an  exposure  of  several  hours  to  light  will  make  positive 
animals  negative,  even  though  the  light  at  the  end  of  the 
period  of  exposure  is  decidedly  fainter  than  it  was  at  the  be- 
ginning (153).  The  positive  reactions  of  the  water  insect 
Ranatra  increase  in  violence  the  longer  the  light  acts ;  on  the 
other  hand,  after  being  kept  in  darkness  for  several  hours, 
Ranatra  is  negative  on  first  being  taken  out  (186).  Daphnias 
kept  in  darkness  for  a  time  become  decidedly  negative  to  dif- 
fused daylight,  whereas  if  kept  in  light  they  would  have  been 


Spatially  Determined  Reactions  177 

positive.  A  sudden  change  in  light  intensity,  either  brighten- 
ing or  darkening,  has  the  effect  of  making  positive  Daphnias 
temporarily  negative  (302). 

Temperature  changes  influence  response  to  light.  The  ob- 
vious suggestion  here  would  be  that  since  increased  tempera- 
ture often  accompanies  increased  intensity  of  light,  animals 
that  are  positively  phototropic  only  up  to  a  certain  degree  of 
illumination  ought  to  become  negative  when  the  temperature 
is  decidedly  raised.  This,  however,  is  by  no  means  always 
the  effect  produced  by  increased  temperature.  Strasburger's 
swarm  spores  became  positive  in  higher  temperatures,  nega- 
tive in  lowered  ones  (392).  Orchestia  agilis,  which  we  have 
just  seen  becomes  temporarily  negative  on  being  brought  from 
strong  into  weak  light,  may  be  made  positive  again  if  the  water 
is  slightly  warmed.  When  the  same  animal  is  dropped  into 
water,  it  becomes  strongly  negative,  but  it  will  show  a  positive 
response  if  the  water  is  heated  almost  to  a  fatal  point  (181). 
On  the  other  hand,  the  copepods  and  annelid  larvae  studied  by 
Loeb  were  made  negative  by  increased,  positive  by  lowered, 
temperature.  Other  crustaceans,  e.g.  Daphnia  (457),  had 
their  responses  to  light  unaffected  by  a  fairly  wide  range 
of  temperature  changes. 

Increasing  or  decreasing  the-devmty  ^Jhe_juiaier  will  also 
affect  phototropism.  In  some  copepods  diluting  the  water 
produced  negative  responses  to  light,  while  increasing  its  den- 
sity brought  about  those  of  the  opposite  sign  (239).  Diluting 
the  water  produced  negative  phototaxis  in  the  larvae  of 
Palaemonetes  (257).  Parker  failed  to  find  any  such  effect  in 
the  case  of  the  copepods  studied  by  him  (304).  W.  Ostwald 
has  called  attention  to  the  possibility  that  "internal  friction'' 
between  the  organism  and  the  medium  may  affect  various 
tropisms.  Freshly  caught  Daphnias,  which  are  negative 
or  indifferent,  quickly  become  positive  if  gelatine  or  quince 


178  The  Animal  Mind 

emulsion  is  added  to  the  water.  Since  they  would  become 
so  in  time  anyway,  Ostwald  thinks  the  mechanical  friction 
of  the  sticky  liquid  simply  acts  as  a  "  sensibilator  "  and  brings 
on  this  positive  phase  sooner  (302). 

Change  in_the  purity  of  the  water,  also  sometimes  produces 
change  of  sign  in  the  response  to  light.  The  amphipod 
Jassa,  negative  in  ordinary  sea  water,  becomes  positive  in 
foul  sea  water  (181).  The  presence  of  chemicals  is  an  influ- 
ence probably  identical  with  the  one  just  mentioned.  Vari- 
ous Crustacea  have  had  the  direction  of  their  reactions 
changed  by  carbonic  or  other  acids,  ammonium  salts,  ether, 
chloroform,  paraldehyd,  and  alcohol  (244).  The  ultra-violet 
rays  will  make  positive  Balanus  larvae  temporarily  negative 

(245). 

The^  statejof  hunger_or  satiety  in  an  animal  must  be  reck- 
oned with:  the  caterpillars  of  Porthesia,  for  example,  are 
decidedly  positive  when  hungry,  much  less  so  when  fed  (236). 
The  slug  Limax  maximus,  ordinarily  negative  to  strong  light, 
is  positive  to  light  of  any  intensity  when  hungry  (135). 

Mechanical  stimulation  is  most  striking  in  its  effect  on  light 
reactions.  Pouchet  in  1872  noted  that  fly  larvae  after  having 
been  shaken  fail  to  display  their  usual  orientation  to  light 
(347).  The  copepod  Temora  longicornisy  usually  negative, 
can  be  made  positive  by  shaking  it  (239).  Very  curious 
phenomena  of  a  similar  nature  have  been  observed  in  the 
case  of  some  Entomostraca.  Certain  individual  specimens 
of  the  ostracod  Cypridopsis  appeared  to  be  decidedly  positive, 
others  negative.  Careful  experimental  analysis  of  the  condi- 
tions revealed  the  following  as  the  true  state  of  affairs.  The 
animals  are  predominantly  negative.  But  contact  with  a 
mechanical  stimulus  has  the  effect  of  making  them  positive; 
thus  a  negative  animal  that  is  picked  up  in  a  pipette,  or  merely 
comes  in  contact  with  the  end  of  the  trough  in  swimming  away 


Spatially  Determined  Reactions  179 

from  the  light,  may  become  positive.  In  course  of  time  such 
a  positive  animal  will  become  negative  of  its  own  accord,  so 
to  speak,  without  further  mechanical  stimulation,  but  such 
stimulation,  if  applied,  makes  it  negative  at  once  (405). 

Similar  experiments  upon  Daphnia  and  Cypris  gave  results 
of  the  same  general  character.  The  strong  positive  tendency 
of  the  former  may,  by  several  times  taking  the  animal  up  in  a 
pipette,  be  made  very  temporarily  negative;  the  opposite 
effect  could  not  be  well  tested  because  of  the  difficulty  of  pre- 
serving the  negative  state  long  enough  to  experiment  on  it. 
In  the  case  of  Cypris,  an  animal  temporarily  negative  could  be 
made  positive  by  picking  it  up,  but  the  positive  phase  could 
not  be  similarly  reversed.  No  other  sudden  stimulus  pro- 
duces the  effect  which  is  thus  induced  by  mechanical  contact 
(449).  And  no  possible  analogy  from  our  own  experience 
suggests  itself ;  the  phenomenon  remains  a  mystery. 

The  effect  of  contact  was  observed  by  Holmes  in  the  ter- 
restrial amphipod  Orchestia  agilis.  The  most  permanent 
phase  of  these  animals  is  positive,  although  they  are  at  rest 
under  seaweed  on  the  beach  by  day.  But  when  they  are 
thrown  into  the  water,  they  become  strongly  negative,  no 
matter  what  the  intensity  of  the  light ;  and  to  a  considerable 
extent  this  effect  is  independent  of  the  temperature  (181).  In 
the  case  of  the  copepod  Labidocera  astiva,  being  picked  up  in 
a  pipette  will  make  the  females,  ordinarily  positive,  negative 
for  a  time.  The  males  are  normally  slightly  negative,  but 
picking  them  up,  instead  of  reversing  this  tendency,  increases 
it  (304).  The  strong  positive  phototropism  of  the  "water 
scorpion"  Ranatra,  an  hemipterous  insect,  may  be  made  nega- 
tive by  handling,  and  especially  by  dipping  in  water  (186). 

Periodical  changes  in  the  sense  of  response  to  light  have 
been  observed  in  animals  subjected  to  periodical  changes  in 
environment.  The  gasteropod  mollusk  Littorina  lives  on 


180  The  Animal  Mind 

the  rocks  of  the  seacoast  in  regions  where  it  is  covered 
with  water  at  high  tide  and  exposed  to  the  air  at  low  tide. 
According  to  the  height  at  which  they  are  found,  some  of  these 
animals  undergo  the  alternations  of  wetness  and  dryness  at 
the  ordinary  tidal  periods,  twice  a  day,  while  others  are 
reached  by  the  water  only  at  the  special  high  tides  occurring 
every  fourteen  days.  Mitsukuri  showed  that  when  the  waves 
of  a  rising  tide  cover  these  mollusks,  they  display  negative 
phototropism  and  seek  shelter  in  rock  cavities ;  while  as  soon 
as  they  are  again  exposed  to  the  air,  their  phototropism  be- 
comes positive  and  they  emerge  in  search  of  food.  Further, 
he  found  that  a  Littorina  whose  phototaxis  was  negative 
could  be  made  positive  by  being  subjected  to  the  action  of  a 
stream  of  water  for  a  time  (275).  Bohn  later  studied  the 
effects  of  placing  black  or  white  screens  near  the  animals  at 
various  angles  to  their  crawling  movements,  and  found  that 
the  black  screens  exerted  an  attractive  influence  at  certain 
times,  the  white  screens  at  others.  These  changes  in  the 
"sense"  of  the  phototropism  correspond  in  time  to  the  oscil- 
lations of  the  tide,  even  though  the  animals  are  studied  in  the 
laboratory;  they  tend  gradually  to  grow  less  pronounced, 
however,  under  such  circumstances.  Further,  the  level  from 
which  the  Littorinas  are  taken  influences  the  nature  of  their 
response  to  light.  Those  from  high  levels,  "which  undergo 
prolonged  and  intense  desiccation,  habitually  move  following 
the  direction  of  the  luminous  field  in  the  negative  sense ;  the 
Littorinas  from  low  levels,  which  undergo  only  short  and 
slight  desiccation,  move,  habitually,  following  the  direction 
of  the  luminous  field  in  the  positive  sense."  The  former 
become  positively  phototropic  at  the  time  of  highest  water, 
the  latter  negatively  phototropic  at  the  time  of  low  water. 
In  all  cases,  the  tendency  is  for  the  animals  to  become  nega- 
tive at  low-water  time.  The  attraction  of  the  dark  screens 


Spatially  Determined  Reactions  181 

represents  that  of  the  dark  surface  of  the  rocks  (55).  Similar 
oscillations  corresponding  to  the  periodicity  of  the  tides  were 
observed  in  the  annelid  Hedista  diver sicolor  (55)  and  in  the 
sea-anemone  Actinia  equina  (65). 

A  further  influence  upon  light  reactions  which  is  doubtless 
involved  in  the  formation  of  the  rhythms  just  described,  has 
been  emphasized  by  Bohn;  namely,  the  "  hydratation "  or 
desiccation  —  the  wetness  or  dryness  —  of  the  tissues.  The 
oscillations  of  Hedista  just  mentioned  may  be  explained  by 
supposing  that  when  the  annelid  is  dry,  light  has  the  power 
of  exciting  muscular  movements.  This  means  that  when  the 
worms  have  accidentally  crept  into  the  shade,  they  stop,  giv- 
ing the  effect  of  negative  photopathy.  If  one  eye  has  its 
illumination  diminished,  there  is  an  inhibition  of  muscular 
activity  on  that  side,  and  consequently  a  turning  in  that  direc- 
tion. At  the  period  of  high  tide,  when  the  muscles  are  wet, 
the  action  of  light  on  the  animal  is  inhibitory  and  the  above 
phenomena  are  reversed.  When  the  Littorinas  observed  by 
Bohn  are  decidedly  moist  or  decidedly  dry,  black  and  white 
screens  exert  an  influence  that  is  proportional  to  their  area; 
the  attractions  and  repulsions  seem  irresistible,  "the  mollusk 
in  the  neighborhood  of  shelter  or  food  continues  on  its  way 
toward  the  screen  as  if  drawn  by  a  fatal  force,  as  if  it  saw  and 
felt  nothing."  But  when  the  tissues  of  its  body  are  in  an 
intermediate  stage  between  "hydratation"  and  desiccation, 
large  screens  have  no  effect  upon  it ;  it  reacts  to  small  objects 
in  its  neighborhood.  "The  animal  seems,  as  it  were,  to  dis- 
engage itself  from  the  influence  of  external  forces,  seems  no 
longer  to  behave  like  a  pure  machine :  it  goes  to  the  stones 
and  seaweed  where  it  may  find  shelter  and  nourishment,  as 
if  it  saw  and  was  conscious  of  them"  (55). 

The  state_ofjrest  or  movement^  still  another  factor.  The 
"mourning  cloak"  butterfly,  ^Vanessa  antiopa,  on  coming  to 


1 82  The  Animal  Mind 

rest  in  bright  sunlight,  orients  itself  with  the  head  away  from 
the  light.  When  it  moves,  on  the  other  hand,  it  flies  toward 
light  of  any  intensity  (307).  Bohn  also  has  noted  that  cer- 
tain butterflies  orient  themselves  when  alighted  in  such  a 
way  that  the  posterior  part  of  the  eyes  is  toward  the  light. 
When  in  this  position  there  is  a  tendency  for  the  wings  to  be 
spread  apart,  while  when  the  insect  is  facing  the  light  the 
wings  are  closely  folded  (55).  The  effect  on  the  wings  was 
noted  in  Vanessa  also,  and,  it  is  suggested,  may  have  some 
function  in  bringing  the  sexes  together  (307).  The  pomace 
fly  when  at  rest  is  not  oriented  at  all.  Light  exerts  upon  it 
merely  the  effect  of  stimulating  it  to  movement,  a  "kinetic," 
not  a  directive,  effect.  When  the  movement  has  been  started, 
however,  it  is  directed  toward  the  light :  positive  phototaxis 
appears.  But  owing  to  the  kinetic  influence  of  the  light, 
when  the  insects  have  been  long  exposed  to  sunlight  they  tend 
to  come  to  rest  in  the  more  shaded  portions,  with  their  heads 
away  from  the  light,  for  this  is  the  position  in  which  they 
are  least  stimulated  to  movement.  The  kinetic  effect  in- 
creases with  the  intensity  of  the  light,  but  its  "  directive 
effect,"  through  which  orientation  is  secured  after  the  move- 
ment is  started,  was  at  least  in  one  case  lost  under  intense 
light  (74). 

The  background,  finally,  sometimes  determines  the  sense 
of  the  reaction.  Keeble  and  Gamble  found  that  while  the 
crustacean  Hippolyte  varians  would  move  toward  the  light 
whether  it  was  on  a  white  or  black  background,  Macromysis 
inermis  was  negative  on  a  white  ground  and  positive  on  a 
black  ground  (218). 

§  71.   Mutual  Influence  of  Light  and  Gravity  Orientations 

Orientation  to  light  and  orientation  to  gravity  are  not  with- 
\    put  mutual  influence  in  determining  the  behavior  of  an 


- 


Spatially  Determined  Reactions  183 

animal.  Supposed  instances  of  this  have  been  noted  in  the 
case  of  the  periodically  changing  geotropism  of  Convoluta 
roscoffensis  (140)  and  in  the  copepods  observed  by  Esterly 
(113).  The  relations  of  gravity  and  light  responses  in  the 
larvae  of  the  squid,  a  cephalopod  mollusk,  seem  to  be  as 
follows.  The  larvae  have  a  tendency  to  rise  to  the  surface  of 
the  water  both  in  darkness  and  in  light,  suggesting  negative 
geotropism.  Two  test  tubes  were  arranged  by  Loeb,  one 
lying  horizontally  and  at  right  angles  to  a  window,  the  other 
inclined  at  an  angle  of  45  degrees  from  the  upright  position, 
and  with  the  upper  end  directed  away  from  the  window. 
Larvae  were  placed  in  both  tubes ;  those  in  the  former  showed 
positive  phototaxis  by  collecting  at  the  end  nearest  the  win- 
dow, but  those  in  the  latter  gave  evidence  that  their  negative 
geotropism  was  stronger  than  their  positive  phototaxis  by 
rising  to  the  upper  end,  although  it  was  farthest  from  the 
source  of  light  (239).  It  is  not  usual  for  geotropism  thus  to 
come  off  victorious  in  a  contest  with  other  tendencies.  Jen- 
nings says,  "  As  a  general  rule  the  reaction  to  gravity  is  easily 
masked  by  reactions  to  other  stimuli  "  (211,  p.  150).  In  the 
mollusks  observed  by  Bohn,  the  tendency  in  ascending  or 
descending  the  rocks  is  to  orient  the  body  in  the  line  of  the 
greatest  slope.  When  light  and  gravity  are  acting  together 
upon  the  animal,  its  movement  seems  to  be  a  resultant  of  the 
two,  but  if  the  mollusk  is  made  to  move  on  a  vertical  plane, 
gravity  thus  exerting  its  maximal  force,  the  influence  of  the 
light  disappears  altogether;  and  if  the  animal  is  put  in  an 
upside-down  position  by  further  tipping  of  the  surface,  the 
sense  of  its  phototropism  is  reversed;  that  is,  it  may  be 
repelled  instead  of  attracted  by  a  dark  screen  (55). 

A  curious  tendency  has  been  noted  by  many  observers  in 
insects  with  both  eyes  blinded ;  namely,  to  fly  straight  up  into 
the  air.  Forel  thought  they  did  so  because  in  no  other 


184  The  Animal  Mind 

direction  could  they  escape  obstacles  (130) ;  but  this  fact  they 
would  have  to  learn  by  experience,  for  which,  in  some  cases 
at  least,  they  do  not  take  time.  Plateau  believed  the  rising 
into  the  air  was  due  to  sensations  produced  by  the  action  of 
the  light  on  the  surface  of  the  body,  leading  the  insects  in 
the  direction  of  the  strongest  light,  which  usually  comes  from 
above.  He  supported  this  view  by  showing  experimentally 
that  a  blinded  insect  would  not  rise  if  set  free  at  night,  while 
on  the  other  hand,  if  liberated  in  a  lighted  room,  it  would,  in 
spite  of  the  blinding,  fly  toward  the  light  or  the  lightest  part 
of  the  ceiling  (332,  334).  In  the  butterfly  Vanessa,  Parker 
thinks  the  rising  due  to  negative  geotropism,  as  the  insect 
flew  upward  in  a  darkened  room  (307).  Axenfeld  suggested 
that  it  might  be  caused  by  light  penetrating  the  integument 
of  the  head  (7). 

§  72.    The  Psychic  Aspect  of  Orientation  to  Light 

What  shall  be  said  of  the  psychic  aspect  of  all  this  complex 
mass  of  facts  regarding  the  orientation  of  animals  to  light? 
If  such  orientation  occurs  in  some  animals  by  the  immediate 
action  of  light  on  the  body  tissues,  either  by  virtue  of  the  direc- 
tion of  its  course  through  them,  or  by  the  relative  effects  on 
the  motor  apparatus,  at  symmetrical  points,  of  stimulations 
differing  in  intensity,  there  is  no  analogy  for  this  in  our  own 
experience.  We  are  not  pulled  about  into  line  by  the  direct 
action  of  light  on  our  bodies,  and  we  cannot  imagine  what  the 
conscious  accompaniment  of  such  a  process  would  be.  If 
orientation  occurs  through  the  giving  of  a  negative  reaction 
whenever  the  body  chances  to  move  out  of  the  oriented  posi- 
tion, we  may  conjecture  that  the  negative  reaction  is,  here  as 
elsewhere,  accompanied  by  unpleasant  consciousness ;  whether 
also  by  a  specific  visual  sensation  will  be  evidenced  by  the 
existence  of  a  sense  organ  or  by  any  other  of  the  arguments 


Spatially  Determined  Reactions  185 

mentioned  in  Chapter  IV.  If  the  effect  of  light  is  merely 
"  kinetic,"  causing  no  orientation,  but  movement  about  until 
the  animal  chances  to  come  into  the  shadow,  vague  restless- 
ness or  uneasiness  is  the  human  experience  most  closely 
resembling  its  possible  conscious  accompaniment.  In  none 
of  these  cases  does  spatial  perception  appear  to  be  concerned. 
Where,  however,  the  response  to  light  depends  upon  the  eyes, 
the  accompanying  psychic  process  may  have  a  spatial  char- 
acter. Even  though  the  eyes  do  not  give  clear  images,  if  the 
reaction  is  determined  by  the  greater  intensity  of  illumination 
on  one  eye  than  on  the  other,  it  is  possible  that  the  visual  field 
present  to  the  animal's  consciousness  may  contain  gradations 
of  intensity  arranged  side  by  side  in  a  spatial  pattern.  An 
important  advance  from  mere  phototropism  to  visual  space 
perception  is  made,  according  to  Ra"dl,  when  an  animal's  eyes 
are  oriented  by  "a  dark  point  in  light  space"  rather  than  by 
"  a  bright  point  in  dark  space,"  but  the  conditions  that  render 
such  orientation  possible  he  does  not  attempt  to  define,  other 
than  by  suggesting  that  they  are  connected  with  the  structure 
of  the  eye  itself  (358). 

§  73.    Orientation  to  Other  Forces 

One  force,  which,  as  was  noted  in  Chapter  III,  produces 
orientation,  namely,  the  electric  current,  we  shall  leave  out 
of  account.  It  is  not  a  stimulus  to  which  animals  are  nor- 
mally subject,  and  though  its  action  on  living  matter  is  of 
great  interest  to  the  physiologist,  the  comparative  psycholo- 
gist's difficulty  in  finding  a  psychic  interpretation  for  the  facts 
may  justify  setting  them  aside.  Similar  considerations  apply 
to  orientation  to  centrifugal  force.  There  remain  the  orien- 
tations that  have  been  termed  respectively  "  rheotropism  " 
and  "  anemotropism,"  responses  to  currents  of  water  and  to 
currents  of  air. 


1 86  The  Animal  Mind 

The  tendency  shown  by  many  aquatic  animals  to  orient 
themselves  with  head  up-stream,  and  to  swim  against  the  cur- 
rent, was  formerly  thought  to  be  a  response  to  the  pressure 
exerted  by  the  current  —  a  reaction  leading  the  animal  to  re- 
sist pressure.  Lyon,  however,  pointed  out  that  this  explana- 
tion assumes  rheotropism  on  the  animal's  part.  It  is  because 
the  animal  opposes  the  current  that  the  current  exerts  any 
pressure.  If  it  merely  allowed  itself  to  be  carried  passively 
along,  and  if  the  current  surrounding  the  animal  flowed  with 
uniform  velocity  in  all  its  parts,  no  stimulus  whatever  could 
be  exerted  by  the  water  pressure  (254).  It  seems  probable 
that  eyeless  animals  do  not,  as  a  matter  of  fact,  orient  them- 
selves against  a  current  of  this  sort,  and  that  rheotropism  in 
their  case  occurs  when  a  current  of  unequal  velocity  disar- 
ranges their  movements,  or  when  they  are  in  contact  with  a 
solid  body.  Thus  Jennings  has  suggested  that  in  Parame- 
cium  the  reaction  is  due  to  the  fact  that  unless  the  animal  has 
its  head  to  the  current,  the  flow  of  the  latter  will  interfere 
with  the  normal  backward  stroke  of  the  cilia,  causing  negative 
reactions  until  the  disturbance  is  removed  by  proper  orienta- 
tion (211,  p.  74).  In  animals  with  eyes,  however,  there  is 
reason  to  think  that  apparent  rheotropism  is  largely  an  affair 
of  vision.  Lyon's  theory  of  rheotropism  in  fishes  is  that  the 
fish  orients  itself  and  swims  in  such  a  way  that  its  surround- 
ings, the  bottom  of  the  stream,  for  example,  shall  appear  to  the 
sense  of  sight  to  be  at  rest,  an  hypothesis  which,  as  we  shall 
see,  was  adopted  by  R£dl  to  explain  the  "hovering"  of  insects 
in  one  place  (355).  Lyon  supports  it  by  experiments  where 
the  bottom  or  sides  of  the  aquarium  were  caused  to  move  in 
the  absence  of  any  current  in  the  water,  and  the  fish  was 
found  to  follow  them.  When  the  fish  was  placed  in  a  re- 
volving glass  cylinder,  it  followed  the  revolutions,  although 
there  was  a  slow  current,  of  course,  in  the  same  direction, 


Spatially  Determined  Reactions  187 

against  which,  on  the  pressure  theory,  the  fish  should  have 
moved.  Still  more  decisive  was  the  experiment  where  young 
fish  were  placed  in  a  corked  bottle  full  of  water  which  was 
submerged  and  put  near  a  wall  covered  with  algae.  When  the 
bottle  was  moved  in  one  direction,  all  the  fish  went  to  the 
opposite  end,  although  no  current  could  have  been  produced. 
Again,  a  wooden  box  with  ends  of  wire  netting,  the  bottom 
covered  with  gravel  and  the  sides  with  seaweed,  was  used; 
fish  (Fundulus)  were  placed  in  it,  and  the  box  was  held 
lengthwise  in  a  strong  current.  The  fish  oriented  them- 
selves, but  as  soon  as  the  box  was  released  and  allowed  to 
float  away,  they  lost  their  orientation,  though  their  relation 
to  the  current  was  in  no  way  altered.  Blind  fish,  Lyon  found, 
oriented  themselves  by  touch,  sinking  to  the  bottom.  There 
does,  however,  appear  to  be,  in  some  cases,  a  genuine  pres- 
sure reaction  to  current,  for  when  water  is  rushing  through  a 
small  hole  into  a  tank  containing  blind  fish,  they  keep  their 
heads  to  the  current  without  touching  anything.  Here  the 
different  parts  of  the  stream  have  different  velocity,  and  pres- 
sure stimuli  are  actually  applied  to  the  skin.  There  must 
be  pressure  reaction,  also,  when  fish  actually  swim  up-stream 
instead  of  merely  maintaining  their  places  against  a  current 
(155).  Such  a  reaction  was  displayed,  probably,  by  some 
shrimps  which,  being  in  the  water  with  the  fish  in  the  revolv- 
ing tank  experiment,  did  swim  against  the  current  instead  of 
with  it  (254). 

Some  very  interesting  behavior  touching  on  this  same  point 
was  observed  by  Garrey  in  a  school  of  the  little  fish  called 
sticklebacks.  He  noted  that  if  any  object  was  moved  along 
the  side  of  the  aquarium  containing  them,  the  whole  school 
would  move  along  a  parallel  line  in  the  opposite  direction. 
If  an  individual  fish  happened  to  be  heading  directly  toward 
the  object,  it  would  turn  in  the  opposite  direction  from  the 


1 88  The  Animal  Mind 

one  in  which  the  object  was  moved ;  if  it  was  heading  some- 
what in  the  opposite  direction  already,  it  would  turn  farther 
in  that  direction  until  parallel  with  the  object's  line  of  motion ; 
if  it  was  heading  somewhat  in  the  same  direction  as  the  object, 
it  would  "back  off  hesitatingly,"  and  reverse  itself  by  a  turn 
in  either  direction,  usually  taking  the  way  around  toward 
which  it  was  already  partially  headed,  if  the  object  was  rapidly 
moved,  but  the  other  way  around  if  the  object's  motion  was 
slow.  At  first  sight  this  behavior  seems  to  display  an  instinct 
precisely  opposite  to  that  of  keeping  the  visual  field  constant. 
Yet  the  sticklebacks,  when  placed  in  a  cylindrical  glass  tank 
inside  of  a  black  and  white  striped  vessel,  moved  with  the 
latter  when  it  moved,  proving  that  they  possessed  the  usual 
tendency  shown  by  Lyon  to  be  involved  in  rheotropism. 
Garrey  points  out  that  movement  in  the  opposite  direction  is 
produced  not  when  the  whole  visual  field  moves,  but  when  it 
is  at  rest,  and  one  object  in  it  moves.  Can  it  be,  he  asks,  that 
the  moving  object  "fixes  the  attention"  of  the  fish  and  pro- 
duces an  apparent  motion  of  the  background  in  the  opposite 
direction,  which  motion  the  fish  follows?  (141.) 

Rheotropism  in  water  arthropods  may  be  similarly  ac- 
counted for,  and  in  the  opinion  of  Ra"dl,  this  same  tendency 
explains  the  habit  swarms  of  insects  have  of  hovering  over  the 
same  place,  a  phenomenon  which  Wheeler  thought  might  be 
due  to  odors  emanating  from  the  soil  (435).  Insects  will  often 
be  found  to  follow  an  object  over  or  under  which  they  are 
grouped  in  the  air,  if  it  be  moved  (355).  Swarms  of  insects 
may  be  noted  in  the  air  over  a  country  road,  following  its 
windings  and  apparently  oriented  by  the  contrast  between 
the  road  and  the  dark  banks  on  either  side.  When,  however, 
resting  insects  turn  so  as  to  keep  their  heads  to  the  wind,  the 
reaction  is  evidently  really  due  to  the  wind  and  not  to  their 
visual  surroundings  (370).  Probably  the  disturbance  to  their 


Spatially  Determined  Reactions  189 

wings  produced  by  any  other  position  causes  them  to  rest  only 
in  the  "head-on"  orientation. 

The  responses  of  animals  to  different  intensities  of  heat 
seem  not  to  involve  a  definite  orientation  of  the  body.  A  tem- 
perature above  the  optimum  produces  wandering  movements, 
which  cease  when  the  animal  happens  to  reach  the  proper 
temperature  (265,  268,  457).  If  we  were  to  adopt  the  ter- 
minology applied  to  light  reactions,  we  should  say  that  ther- 
mopathy  rather  than  thermotaxis  is  the  rule. 


CHAPTER  IX 

SPATIALLY  DETERMINED  REACTIONS  AND  SPACE  PERCEPTION 

(continued) 

§  74.    Class  III:   Reactions  to  a  Moving  Stimulus 

SPECIALIZED  response  to  a  stimulus  in  motion,  that  is, 
one  which  successively  affects  several  neighboring  points  on 
a  sensitive  surface,  is  also  frequently  met  with  in  animal  be- 
havior. Its  usefulness  is  obvious :  a  stimulus  in  motion  is 
very  commonly  a  living  creature,  hence  either  an  enemy  or 
food.  In  any  case  it  must  be  reacted  to  with  extreme  prompt- 
ness. Reactions  of  this  class  may  be  distinguished  as  tactile 
or  visual  according  as  the  moving  stimulus  is  mechanical  or 
photic. 

We  find  good  examples  of  specialized  reactions  to  motile 
touch  in  the  ccelenterates.  The  sea-anemone,  Aiptasia,  gives 
its  most  violent  reaction,  involving  all  the  tentacles  at  once, 
when  touched  by  a  moving  object  (291).  The  medusa 
Gonionemus  makes,  in  the  case  of  a  moving  mechanical 
stimulus,  its  single  exception  to  the  rule  of  responding  by 
the  feeding  reaction  to  edible  substances  only.  The  tentacles 
will  be  wound  corkscrew  fashion  about  a  glass  rod  drawn 
across  them,  they  bend  in  toward  the  mouth,  and  the  bell 
margin  bearing  them  contracts ;  the  feeding  reaction  goes  no 
further,  however.  But  the  response  is  differentiated  from  that 
to  any  other  form  of  stimulation  by  its  greater  speed :  the 
reaction  time  is  from  .3  to  .35  of  a  second,  compared  with  .4  to 
.  5  of  a  second  for  other  stimuli  (451).  Special  vigor  and  speed 

190 


Spatially  Determined  Reactions  191 

generally  characterize  reactions  to  contact  with  moving  ob- 
jects. In  eliciting  the  scratch-reflex  of  dogs,  an  object  drawn 
along  the  skin  is  decidedly  more  effective  than  one  pressed 
against  the  skin  for  the  same  length  of  time  (382,  p.  184). 
The  physiological  effect  is  probably,  Sherrington  says,  the 
same  as  that  involved  in  the  "summation"  of  successive  slight 
stimuli  applied  at  the  same  point.  As  is  well  known,  the  latter 
will  bring  about  a  response  of  considerable  violence,  though 
each  one  acting  alone  would  apparently  be  without  effect. 

Is  it  likely  that  these  responses  to  moving  stimuli  in  contact 
with  the  skin  involve  the  perception  of  movement  as  a  form 
of  space  perception;  that  is,  a  perception  of  the  successive 
positions  occupied  by  the  stimulus  and  their  relative  direc- 
tion ?  I  think  we  may  say  that  they  probably  do  not,  in  the 
lower  animal  forms  at  least.  And  a  chief  reason  for  saying 
so  lies  in  the  fact  that  the  reactions  are  so  rapid.  To  perceive 
the  spatial  relations  of  stimuli,  or  any  other  relations,  is  a 
process  not  favored  by  great  speed  of  response.  The  quicker 
the  reaction,  the  less  clear  the  perception  of  its  cause :  such 
seems  to  be  the  general  law.  The  sensation  accompanying 
contact  with  a  moving  object  may  differ  in  intensity  from  that 
accompanying  a  resting  stimulus ;  it  may,  in  the  lower  forms, 
differ  qualitatively  in  some  way  not  represented  in  our  own 
experience,  but  it  can  hardly  be  connected  with  the  more 
complex  psychic  processes  involved  in  any  form  of  space  per- 
ception. 

In  vision,  also,  there  are  special  arrangements  for  reacting 
to  moving  stimulation.  The  sensitiveness  of  many  animals 
to  changes  of  light  intensity,  although  not  a  direct  adaptation 
to  the  spatial  characteristics  of  a  stimulus,  serves  the  same 
purpose,  for  changes  in  light  intensity  are  oftenest  brought 
about  by  objects  in  motion.  In  the  mollusk  Pecten  varius, 
a  transition  from  shadow  vision  to  movement  vision  is  illus- 


192  The  Animal  Mind 

trated :  the  animal  closes  its  shell  when  a  shadow  is  moved  so 
as  to  fall  on  its  eye  spots  in  rapid  succession  (360).  Generally 
speaking,  the  simple  invertebrate  eye,  however,  is  adapted  to 
respond  to  changes  in  light  intensity  rather  than  to  moving 
objects.  Plateau  found  that  caterpillars,  which  have  only 
simple  eyes,  could  see  moving  objects  no  better  than  those  at 
rest  (333),  and  Willem  was  inclined  to  think  snails  saw  rest- 
ing objects  better  than  moving  ones  (441).  On  the  other 
hand,  the  compound  eye  is  specially  formed  to  be  affected  by 
moving  stimuli.  The  crayfish  will  react  to  anything  of  fairly 
good  size  in  motion,  but  is  apparently  unable  to  avoid  sta- 
tionary objects  in  its  path  (21).  The  poor  vision  of  the  com- 
pound eye  for  resting  objects  is  shown  by  the  ease  with  which 
insects  may  be  captured  if  the  movements  of  the  captor  are 
very  slow.  They  may  be  readily  approached,  also,  if  the 
movements  are  all  in  the  line  of  sight,  that  is,  directly  toward 
the  insect,  so  that  successive  facets  of  the  compound  eye  are 
not  affected,  as  would  be  the  case  in  lateral  movements.  Let 
the  reader  try  bringing  the  hand  slowly  straight  down  over 
a  fly,  and  see  how  much  closer  he  can  come  before  the  fly  is 
disturbed  than  he  can  if  the  hand  is  moved  from  side  to  side. 
Plateau,  from  experiments  on  different  orders  of  insects,  con- 
cludes that  "visual  perception  of  movement"  is  best  devel- 
oped in  the  Lepidoptera  (moths  and  butterflies),  Hymen- 
optera  (ants,  bees,  and  wasps),  Diptera  (flies),  and  Odonata 
(dragon-flies);  that  the  distance  at  which  movements  can 
be  seen  does  not  exceed  two  metres,  and  averages  1.5 
metres  for  diurnal  Lepidoptera,  58  cm.  for  Hymenoptera, 
and  68  cm.  for  Diptera  (335). 

It  is  possible  that  response  to  a  moving  stimulus  received 
through  the  eye  may  be  accompanied  by  spatial  perception 
of  movement,  although  if  the  eye  is  compound,  the  experience 
must  differ  from  our  own  visual  movement  perception. 


Spatially  Determined  Reactions  193 

§  75.    Class  IV:   Reaction  to  an  Image 

By  an  image  is  meant  the  perception  of  simultaneously  oc- 
curring but  differently  located  stimuli  as  having  certain  spa- 
tial relations  to  each  other.  Through  its  means,  or  that  of 
the  nervous  processes  underlying  it,  there  arises  the  possi- 
bility of  adapting  reaction  not  merely  to  the  location  of  a 
single  stimulus,  but  to  the  relative  location  of  several  stimuli. 
Responses  may  thus  be  adjusted  not  only  to  the  direction  of 
an  object  but  to  its  form.  On  the  basis  of  such  adjustments 
a  whole  new  field  of  possible  discriminations  is  opened  up. 

The  commonest  arrangement  for  the  production  of  a  visual 
image  is  the  double  convex  lens,  which  collects  the  rays  of 
light  diverging  in  their  reflection  from  an  object  and  brings 
them  together  again  upon  the  sensitive  retina.  The  lenses 
found  in  many  simple  invertebrate  eyes  seem,  however,  very 
ill  adapted  to  the  image-producing  function.  It  is  probable 
that  they  serve  rather  to  intensify  the  effect  of  the  light  rays 
by  bringing  them  together,  than  to  give  a  clear-cut  image 
(293).  In  the  eye  of  certain  invertebrates,  such  as  the  Nauti- 
lus, a  gasteropod  mollusk,  while  there  is  no  lens,  the  opening 
admitting  the  light  rays  is  so  small  that  an  inverted  image 
might  be  formed  through  it,  such  as  may  be  obtained  through 
a  pinhole.  It  is  unlikely,  however,  that  this  eye  is  really  an 
image-producing  organ.  Hesse  includes  under  image-form- 
ing eyes  only  the  camera  or  convex-lens  eye,  the  mosaic  eye, 
and  the  superposition  eye.  The  last  is  a  peculiar  form  of  com- 
pound eye  where  light  can  pass  from  one  section  to  another, 
and  where  the  image  is  formed  by  the  cooperation  of  various 
refracting  bodies  (176). 

The  simplest  and  vaguest  conceivable  visual  image  would 
be  that  of  a  visual  field  whose  different  parts  should  differ 
in  brightness.  An  eye  capable  of  furnishing  indications 


194  The  Animal  Mind 

merely  of  the  direction  from  which  the  greatest  illumination 
comes  might  produce  this  kind  of  an  image,  which  would  of 
course  not  allow  the  perception  of  objects,  only  that  of 
brightness  distribution.  ( We  have  already  seen  that  the  orien- 
tations of  certain  animals  to  light  seem  to  be  produced  through 
a  tendency  to  take  such  a  position  that  the  two  eyes  shall  be 
equally  illuminated.  If  the  two  visual  fields  are  combined  in 
the  case  of  such  animals,  as  they  are  in  our  own  binocular 
vision,  under  ordinary  conditions  the  oriented  position  would 
give  a  visual  field  whose  brightness  is  equal  throughout, 
while  any  other  position  would  give  greater  brightness 
at  one  side  of  the  field.  If  they  are  not  combined,  if  there 
is  no  binocular  vision,  we  cannot  imagine  what  the  resulting 
perception  is.  That  the  direction  from  which  the  light  comes 
influences  ants  in  finding  their  way  is,  we  have  seen,  the  opin- 
ion of  Lubbock  (248 )  and  of  Turner  (408).  It  was  found  not 
to  be  important  to  white  rats  in  learning  a  labyrinth  path  (43i).1 

§  76.    Methods  of  investigating  the   Visual  Image:  the 
Size   Test 

The  presence  of  a  visual  image  that  is  something  more  than 
a  visual  field  of  graded  brightnesses  has  been  tested  by  meth- 
ods which  may  be  divided  into  two  groups :  those  which 
investigate  the  effect  of  stimuli  differing  in  area  but  of  the 
same  intensity,  and  those  which  test  discrimination  of  the 
form  of  objects. 

Bohn's  observations  on  the  mollusk  Littorina  show  that  its 
reactions  are  influenced  by  the  size  of  the  illuminated  or  dark- 
ened surface,  as  well  as  by  the  intensity  of  the  light.  When 
neither  very  wet  nor  very  dry,  Littorina  will  react  to  small 
objects  in  its  neighborhood,  whereas  in  an  extreme  state  of 
"  hydratation "  or  desiccation  it  responds  to  the  attraction  or 
repulsion  of  the  larger  screens  with  fatal  uniformity  (55). 


Spatially  Determined  Reactions  195 

Plateau  attempted  to  test  the  responses  of  certain  Dip- 
tera  to  the  size  of  an  opening  admitting  light,  by  placing  them 
in  a  dark  room,  into  which  light  entered  from  two  sources. 
One  was  a  single  orifice  large  enough  to  let  the  insects  out ; 
the  other  was  covered  with  a  net  whose  meshes  were  too  fine 
to  allow  them  to  pass.  The  amount  of  light  from  the  two 
sources  could  be  made  equal.  When  this  was  done,  the 
insects,  which  were  positively  phototropic,  sought  the  two 
equally  often ;  if  the  light  from  either  was  made  more  intense, 
they  went  to  that  one.  Plateau  concluded  both  that  the  flies 
could  not  see  the  netting  and  that  the  area  of  the  light  source 
did  not  affect  them  (328).  On  the  other  hand,  Parker 
found  that  the  mourning-cloak  butterfly  did  discriminate 
areas,  flying  to  the  larger  of  two  sources  of  equally  intense 
light  (307). 

This  method  of  testing  the  image-forming  power  of  an 
animal's  eyes  has  recently  been  elaborated  by  L.  J.  Cole. 
He  subjected  animals  with  decided  positive  or  negative 
phototropism  to  the  influence  of  two  lights  made  equally 
intense  but  differing  in  area,  one  coming  through  a  piece  of 
ground  glass  41  cm.  square,  the  other  a  mere  point.  Eye- 
less animals,  the  earthworm,  for  example,  reacted  equally 
often  to  each  light.  Animals  whose  eyes  from  their  structure 
have  been  judged  capable  of  perceiving  merely  the  direction 
of  light  rays,  such  as  the  planarian  Bipalium,  confirmed  the 
argument  from  structure  by  showing  little  more  discrimination 
than  the  eyeless  ones.  On  the  other  hand,  animals  with 
well-developed  compound  or  camera  eyes,  for  example  certain 
insects  and  frogs,  did  distinguish  between  the  lights,  going, 
if  positively  phototropic,  toward  the  one  of  larger  area; 
if  negatively  phototropic,  away  from  it  (80). 

Discrimination  of  boxes  differing  in  size,  but  alike  in  form, 
placed  in  a  row  along  a  board,  food  having  been  put  in  one, 


196  The  Animal  Mind 

was  imperfectly  learned  by  two  Macacus  monkeys ;  the  errors 
leaned  in  the  direction  of  taking  the  larger  vessel  (221). 
Raccoons  were  taught  to  distinguish  perfectly  between  two 
cards,  one  6jx6|  inches  square  and  the  other  4^X4^, 
shown  successively.  The  animals  were  to  climb  on  a  box 
for  food  when  the  larger  card  was  shown  and  to  stay  down 
when  the  smaller  one  appeared.  As  we  shall  see  later,  L.  W. 
Cole,  the  experimenter,  thinks  the  learning  gave  evidence  not 
only  of  a  spatial  image,  but  of  a  memory  image  (82). 

One  apparent  effect  of  size  upon  visual  perception  relates 
to  the  distance  at  which  an  object  produces  a  reaction. 
Caterpillars,  for  example,  are  described  as  giving  evidence  of 
seeing  a  slender  rod  extended  toward  them  at  a  distance  of 
about  a  centimeter;  large  masses  they  reacted  to  at  some- 
what greater  distance  (333).  It  is  highly  doubtful  whether 
this  means  that  the  simple  eye  of  the  caterpillar  could  give 
a  perception  of  two  objects  as  differing  in  size  if  they  were 
equally  distant.  Myriapods,  which  make  very  little  use  of 
sight  and  do  not  perceive  their  prey  until  they  touch  it,  give 
evidence  of  seeing  an  obstacle  having  a  rather  broad  surface, 
the  size  of  a  visiting  card,  at  a  distance  of  about  10  cm., 
if  it  is  white  and  reflects  much  light,  or  if  it  is  blue; 
but  not  if  it  is  red,  —  another  indication  of  the  relation  be- 
tween white  and  blue  light,  red  light  and  darkness,  noted  on 
p.  123  (329). 

§  77.   Methods  of  investigating  the   Visual  Image:  the 

Form  Test 

The  second  method  of  studying  visual  images,  that  of 
testing  an  animal's  power  to  discriminate  forms,  has  been 
applied  chiefly  to  the  higher  vertebrates.  Bumblebees,  to  be 
sure,  were  thought  by  Forel  to  evince  a  capacity  to  distin- 
guish a  blue  circle  from  a  blue  strip  of  paper  when  they  had 


Spatially  Determined  Reactions  197 

previously  found  honey  on  a  blue  circle,  even  though  the  two 
had  been  made  to  exchange  places.  They  flew  first  to  the 
place  where  the  blue  circle  had  been,  but  did  not  alight  upon 
the  strip.  Wasps,  also,  according  to  Forel,  distinguished 
among  a  disk,  a  cross,  and  a  band  of  white  paper,  going  first 
to  the  form  on  which  they  had  last  found  honey  (130). 
Various  species  of  birds  were  experimented  on  by  the  method 
of  placing  cards  carrying  simple  designs  over  glasses  covered 
with  gray  paper,  food  being  placed  always  under  the  same 
card.  The  English  sparrow  and  the  cowbird  both  learned  to 
distinguish  a  card  bearing  three  horizontal  bars  and  one 
bearing  a  black  diamond  from  each  other  and  from  plain 
gray  cards.  On  the  other  hand,  the  sparrow,  curiously 
enough,  did  not  succeed  in  discriminating  vessels  of  different 
form ;  the  cowbird  was  not  fully  tested  with  these,  but  gave 
some  evidence  that  it  was  learning  (344,  345) .  Pigeons  were 
only  moderately  successful  in  a  similar  test  (371). 

Many  dogs  have  been  taught  to  distinguish  printed  letters 
on  cards;  Sir  John  Lubbock's  poodle  "Van"  is  a  familiar 
example.  Van  learned  to  pick  out  cards  marked  "Food," 
"Bone,"  "Out,"  "Water,"  and  the  like,  and  to  present  each 
on  its  appropriate  occasion.  It  took  him  ten  days  to  begin  to 
make  the  first  step  of  distinguishing  between  a  printed  card 
and  a  plain  one ;  in  a  month  this  was  perfected  and  in  twelve 
more  days,  when  he  wanted  food  or  tea,  he  brought  the  right 
card  one  hundred  and  eleven  times  and  the  wrong  one  twice. 
The  second  mistake  consisted  in  bringing  the  word  "door" 
instead  of  "food,"  indicating  that  it  was  really  the  look  of  the 
words  that  he  distinguished  (251,  p.  277  f.). 

The  dancing  mouse  could  not  learn  to  distinguish  two  equal 
illuminated  areas  of  different  forms  (469).  Raccoons  learned 
to  discriminate  a  round  card  from  a  square  one  (82).  Thorn- 
dike  taught  the  two  Cebus  monkeys  under  his  observation  to 


198  The  Animal  Mind 

come  down  to  the  bottom  of  the  cage  for  food  when  a  card 
bearing  the  word  "Yes"  printed  on  it  was  exposed,  and  to 
stay  up  when  one  bearing  the  letter  "N"  was  shown.  The 
conditions  seem  to  have  been  complicated,  however,  by  the 
fact  that  the  two  cards  were  not  placed  in  quite  the  same 
position.  Further  tests  with  cards  carrying  various  designs 
showed  varying  degrees  of  capacity  to  distinguish  them  on 
the  part  of  the  monkeys  (397).  Kinnaman  got  negative 
results  with  his  two  Macacus  monkeys  in  attempting  to 
train  them  to  distinguish  cards  such  as  those  used  in  the  later 
experiments  of  Porter  on  birds.  His  monkeys,  however, 
proved  able  to  distinguish  vessels  of  different  forms,  "a  wide- 
mouthed  bottle,  a  small  cylindrical  glass,  an  elliptical  tin 
box,  a  triangular  paper  box,  a  rectangular  paper  box,  and  a 
tall  cylindrical  can.'7  These  vessels  differed  in  size  as  well 
as  in  form  (221). 

Special  evidence  of  the  comparative  development  of  the 
visual  image  in  different  genera  of  ants  is  suggested  by 
Wasmann  to  be  furnished  by  the  facts  of  mimicry.  Certain 
insects  belonging  to  orders  other  than  the  Hymenoptera 
inhabit  ants'  nests,  and  have  in  many  cases  become  more  or 
less  modified  to  resemble  their  hosts.  Wasmann  thinks 
that  these  resemblances,  which  have  been  established  on 
account  of  their  protective  value,  are  in  insects  living  among 
ants  of  well-developed  visual  powers,  such  as  would  deceive 
especially  the  sense  of  sight,  while  in  the  "guests"  of  ants 
whose  vision  is  poor,  the  mimicry  is  adapted  to  produce 
tactile  illusions  (426). 

§  78.   Class  V :  Reactions  adapted  to  the  Distance  of  Objects 
The  factors  that  make  possible  the  perception  of  the  third 
dimension,  depth,  or  distance  outward  from  the  body,  in  in- 
vertebrate animals  are  little  known.    Certain  invertebrates  do 


Spatially  Determined  Reactions  199 

give  evidence  of  the  power  to  judge  distance.  The  hunting 
spiders,  for  example,  which  do  not  make  webs,  but  pursue 
their  prey  in  the  open,  leap  on  it  from  a  distance  of  several 
inches.  Dahl  thinks  distinct  vision  is  limited  to  two  centi- 
meters (88),  and  Plateau  says  capture  is  not  attempted  until 
the  prey  is  within  this  distance  (332).  The  Peckhams, 
however,  tested  a  hunting  spider  by  putting  it  at  one  end  of  a 
narrow  glass  case  sixteen  inches  long,  at  the  other  end  of 
which  a  grasshopper  was  placed.  When  eight  inches  from 
its  victim,  the  spider's  movements  changed,  and  at  four 
inches  the  leap  was  made1  (321). 

Reactions  of  this  character,  where  the  animal  makes  a 
single  movement  adapted  to  the  distance  of  an  object  from  it, 
are  almost  the  sole  evidence  we  can  get  of  accurate  perception 
of  the  third  dimension.  The  alleged  performance  of  the 
jaculator  fish,  which,  as  described  by  Romanes,  "shoots  its 
prey  by  means  of  a  drop  of  water  projected  from  the  mouth 
with  considerable  force  and  unerring  aim,"  the  prey  being 
"some  small  object,  such  as  a  fly,  at  rest  above  the  surface 
of  the  water,  so  that  when  suddenly  hit  it  falls  into  the  water," 
would  involve  distance  perception  (364,  p.  248).  The  catch- 
ing of  insects  on  the  wing  by  various  amphibians,  reptiles, 
and  birds  has  the  same  significance.  A  salamander  cau- 
tiously stalking  a  small  fly  will  not  strike  until  it  gets  within 
a  certain  distance.  In  Necturus  and  in  other  animals  the 
pause  just  before  snapping  at  food  has  been  suggested  to  be 
for  the  purpose  of  proper  fixation  (438). 

Yerkes's  tests  of  the  so-called  "sense  of  support"  in  tor- 
toises indicate  some  power  of  estimating  distance  by  vision 

1  Porter  observed  that  the  distance  at  which  spiders  of  the  genera  Argiope 
and  Epeira  could  apparently  see  objects  was  increased  six  or  eight  times  if 
the  spider  was  previously  disturbed  by  shaking  her  web  (346).  This,  of 
course,  does  not  refer  to  the  power  to  judge  distance. 


2OO  The  Animal  Mind 

in  these  animals.  He  experimented,  it  will  be  remem- 
bered, with  individuals  belonging  to  three  classes:  land- 
dwelling,  water-dwelling,  and  amphibious.  The  first  men- 
tioned would  crawl  off  the  edge  of  a  board  30  centimeters 
above  a  net  of  black  cloth  only  with  much  reluctance  when 
their  eyes  were  uncovered ;  when  blindfolded  they  would  not 
move  at  all.  The  water  tortoises  plunged  off  without  hesita- 
tion from  a  height  of  30  centimeters,  but  hesitated  slightly 
at  90  centimeters,  although  some  individuals  would  take  the 
plunge  at  once  even  from  a  height  of  180  centimeters.  When 
blindfolded,  all  of  the  water  tortoises  rushed  off  at  any  height. 
The  land-and-water-dwelling  tortoises  hesitated  at  30  centi- 
meters and  at  90  centimeters  showed  a  conflict  of  impulses, 
trying  to  catch  themselves,  before  launching  off.  When 
blindfolded  they  would  not  leave  the  board  at  all,  though  they 
moved  about  upon  it  freely  (459). 

Some  of  the  most  important  conditions  of  distance  per- 
ception in  our  own  experience  are  lacking  in  the  lower 
vertebrates  and  in  invertebrates.  Stereoscopic  vision,  the 
appearance  of  solidity  given  to  objects  by  the  fact  that  the 
visual  fields  of  the  two  eyes  combine,  thus  producing  blending 
of  two  slightly  different  views  of  the  object  looked  at,  has 
been  held  to  be  dependent  on  the  partial  crossing  of  the  op- 
tic nerves  on  their  way  to  the  brain,  whereby  each  retina  sends 
nerve  fibres  to  both  hemispheres  of  the  brain.  This  arrange- 
ment does  not  appear  in  the  animal  kingdom  below  the  birds ; 
whatever  function  it  plays  in  space  perception  is,  then, 
absent  from  reptiles,  amphibians,  fish,  and  invertebrates. 
Certainly  stereoscopic  vision  cannot  exist  in  animals  whose 
eyes  are  so  placed  that  the  same  object  cannot  be  seen  by 
both,  as  is  the  case  with  most  fishes.  -In  birds,  whose  eyes 
are  situated  too  far  toward  the  sides  of  the  head  for  the 
same  object  to  cast  its  images  on  the  foveas  or  centres  of  the 


Spatially  Determined  Reactions  201 

two  retinas,  there  appears  to  be  a  secondary  fovea  in  each 
eye,  so  placed  as  to  suggest  that  it  serves  binocular  vision, 
while  the  primary  fovea  is  used  for  monocular  vision.  Con- 
vergence, the  turning  of  the  eyes  toward  each  other  to  bring 
the  two  images  of  an  object  on  the  central  part  of  the  retinas, 
which  is  an  important  aid  to  human  estimations  of  distance, 
is  also  necessarily  lacking  in  animals  without  binocular 
vision.  A  third  factor  in  our  own  perceptions  of  distance, 
the  accommodation  of  the  crystalline  lens,  that  is,  the  altera- 
tion of  its  convexity  through  the  pull  of  the  accommodation 
muscle  to  enable  it  to  focus  objects  at  different  distances, 
has  been  carefully  studied  in  connection  with  the  lower 
animals  by  Beer.  \  Through  experiments  on  the  refractive 
powers  of  eyes  dissected  from  the  dead  animal,  he  reached  the 
conclusion  that  no  invertebrates  but  cephalopods  have  the 
power  of  accommodation.^  It  is  rudimentary  or  lacking  also  in 
some  members  of  the  fish,  lizard,  crocodile,  snake,  and  mam- 
mal families.  In  cephalopods,  fishes,  amphibians,  and  most 
reptiles,  the  process  of  accommodation  does  not  involve  a 
change  in  the  form  of  the  lens,  but  an  alteration  in  the  dis- 
tance between  the  lens  and  the  retina.  The  device  of  in- 
creasing the  curvature  of  the  lens  for  vision  of  near  objects 
appears  first  in  certain  snakes,  and  is  found  throughout  the 
higher  vertebrates  (18). 

Where  accommodation  does  not  exist,  as  in  most  inverte- 
brates, it  is  possible  to  trace  other  arrangements  for  adapting 
vision  to  the  distance  of  the  object  seen.  Thus  in  com- 
pound eyes,  part  of  the  eye  may  be  adapted  to  near  vision  and 
part  to  far  vision.  This  is  suggested  by  the  fact  that  some 
of  the  little  tubes,  or  ommatidea,  of  which  the  compound  eye 
is  composed,  diverge  from  each  other  by  a  less  angle  than 
others,  indicating  that  they  are  suited  to  the  reception  of  more 
nearly  parallel  rays.  In  insects  with  both  simple  and  com- 


2O2  The  Animal  Mind 

pound  eyes  one  form  may  be  used  for  near  and  one  for  far 
vision.  Spiders  appear  to  have  the  principal  eyes  adapted 
for  far  vision  and  the  auxiliary  eyes  for  near  vision,  while  one 
spider,  Epeira,  has  part  of  the  hinder  median  eye  adapted  to 
each  (176). 

§  79.  Some  Theoretical  Considerations 
The  temptation  is  strong  to  speculate  upon  the  essential 
nature  of  the  conditions  which  make  possible  true  space 
perception,  the  simultaneous  experiencing  of  sensations  that 
are  referred  to  different  points  in  space.  Such  speculation 
must  be  of  the  most  tentative  description,  yet  the  following 
suggestions  seem  not  wholly  unwarranted  by  the  facts.  For 
one  thing,  it  looks  probable  that  the  ability  to  suspend  im- 
mediate reaction  is  essential  to  space  perception.  Can  a 
spatial  complex  of  sensations  occur  in  the  experience  of  an 
organism  unless  that  organism  is  capable  of  receiving  a  number 
of  stimuli  on  a  sensitive  surface  and  of  suspending,  for  a 
brief  period  at  least,  all  reaction  ?  Let  us  take  as  an  example 
of  such  a  complex  a  visual  field,  within  which  different  color 
and  brightness  qualities  are  arranged  in  definite  order,  some 
above,  some  below,  some  to  the  right,  others  to  the  left. 
Could  such  a  balance  of  tendencies  to  move  the  eye  as  is 
involved  in  the  simultaneous  perception  of  a  number  of 
elements  preserving  regular  space  relations  to  each  other  have 
been  brought  about  unless  no  single  one  of  the  tendencies  were 
irresistible?  One  can  readily  imagine  an  eye  functioning 
in  such  a  way  that  every  stimulation  of  it,  though  occasioned 
by  rays  from  several  different  directions  acting  simultaneously, 
should  issue  at  once  in  a  resultant  movement.  Would  not  the 
accompanying  consciousness  be  a  single  resultant  sensation, 
rather  than  a  complex  of  spatially  ordered  elements?  It  is 
a  good  deal  easier,  of  course,  to  ask  than  to  answer  such 
questions. 


Spatially  Determined  Reactions  203 

/'  Again,  the  power  of  getting  true  spatial  images  seems  to 
be  bound  up  closely  with  the  power  of  moving  the  sensitive 
surface.  We  get  our  best  tactile  space  perceptions  through 
active  touch,  involving  movement  of  the  hands  and  fingers; 
our  visual  space  perceptions  are  profoundly  influenced  by  eye 
movements.  Where  the  movements  of  an  animal's  body  as  a 
whole  are  very  rapid,  as  in  the  case  of  winged  insects,  this 
fact  may  compensate  for  the  immovability  of  its  eyes. 
Forel,  as  we  have  seen,  thinks  that  insects  which  can  explore 
objects  by  moving  the  antennae,  bearing  the  organs  of  smell, 
over  them,  may  have  smell  space  perceptions,  such  as  are  un- 
known to  our  experience ;  they  may  perceive  the  shape  and 
size  of  odorous  patches  as  we  could  do  if  our  organs  of  smell 
were  on  our  hands  (132).  Now,  movement  of  a  sense  organ 
brings  about  the  same  result  that  movement  of  a  stimulus 
across  a  resting  sense  organ  does ;  that  is,  the  stimulus  affects 
different  points  of  the  sensitive  surface  in  succession.  But 
the  vital  significance  of  the  two  is  quite  different ;  movement 
of  an  object  across  a  resting  sense  organ  means  very  likely 
that  the  object  is  alive ;  it  must  be  instantly  reacted  to,  and  the 
speed  of  the  reaction  is  unfavorable  to  the  formation  of  a  true 
space  perception.  Movement  of  the  sense  organ,  however, 
gives  a  series  of  impressions  on  successive  points  of  the  sensi- 
tive surface,  from  a  resting  object.  While  the  sense  organ  is 
being  moved,  it  is  probable  that  other  reactions  of  the  animal 
will  be  suspended.  Whether  any  part  in  the  formation  of 
that  complex  conscious  content  which  we  call  a  spatial 
image,  consisting  of  different  sensations  simultaneously 
apprehended,  is  played  by  the  "lasting  over"  of  the  impres- 
sions on  one  sensitive  point  after  the  stimulus  has  passed  on  to 
the  next,  a  phenomenon  which  we  find  both  in  touch  and  in 
sight  sensations,  it  is  impossible  to  say.  (^We  are,  however, 
apparently  justified  in  the  statements  that  the  essence  of 


2O4  The  Animal  Mind 

space  perception,  as  distinct  from  other  conscious  processes 
that  may  accompany  spatially  determined  reactions,  is  the 
presence  of  an  image  in  the  sense  above  defined,  and  that  a 
movable  sense  organ  is  an  important  condition  for  the  pro- 
duction of  such  an  image.) 


CHAPTER    X 

THE  MODIFICATION  OF  CONSCIOUS  PROCESSES  BY 
INDIVIDUAL  EXPERIENCE 

(THE  reactions  of  animals  to  stimulation  show,  as  we  re- 
view the  various  animal  forms  from  the  lowest  to  the  highest, 
increasing  adaptation  to  the  qualitative  differences  and  to  the 
spatial  characteristics  of  the  stimuli  acting  upon  them.  It  is 
therefore  possible  to  suppose  that  the  animal  mind  shows 
increasing  variety  in  its  sensation  contents,  and  increasing 
complexity  in  its  spatial  perceptions.  But  besides  this 
advance  in  the  methods  of  responding  to  present  stimu- 
lation, the  higher  animals  show  in  a  growing  degree  the 
influence  of  past  stimulation.  While  a  low  animal  may 
apparently  react  to  each  stimulus  as  if  no  other  had  affected 
it  in  the  past,  one  somewhat  higher  may  have  its  reaction 
modified  by  the  stimulation  which  it  has  just  received.  An 
animal  still  more  highly  developed  may  give  evidence  of  being 
affected  by  stimuli  whose  action  occurred  some  time  before ; 
and  finally,  in  certain  of  the  vertebrates,  perhaps,  as  in  man, 
conduct  may  be  determined  by  the  presence  in  consciousness 
of  a  memory  idea  representing  a  past  stimulus.  "  Learning 
by  experience,"  or  "associative  memory,"  as  we  saw  in  Chap- 
ter II,  has  been  regarded  as  the  evidence  par  excellence  of  the 
existence  of  mind  in  an  animal.  That  it  does  not  serve  this 
purpose  to  entire  satisfaction  was  also  pointed  out  in  that 
earlier  chapter,  and  will  be  more  clearly  apparent  as  we  survey 
in  the  following  pages  the  various  ways  in  which  an  organ- 

205 


206  The  Animal  Mind 

ism's  past  experience  may  modify  its  behavior,  asking  each 
time  what  the  possible  conscious  aspect  of  the  modification 
may  be. 

§  80.    Absence  of  Modification 

In  the  first  place  there  presents  itself  for  consideration  the 
case  of  animals  that  meet  a  situation  by  repeating  the  same 
reaction  over  and  over  again.  For  example,  Paramecium 
encounters  an  obstacle  in  its  path.  It  performs  the  only 
reaction  in  its  power,  the  avoiding  reaction;  it  darts  back- 
ward, rolls  to  one  side,  and  proceeds  forward  at  an  acute 
angle  to  its  former  course.  Suppose  that  the  obstacle  is  so 
large  that  the  animal  strikes  it  again.  The  negative  reaction 
is  repeated  and  again  repeated  if  need  be  until  the  course 
is  sufficiently  altered  to  carry  the  Paramecium  clear  of  the 
obstacle.  To  behavior  of  this  sort  Jennings  has  extended 
the  term  "trial  and  error"  (206,  p.  237).  The  expression 
was  first  used  by  Lloyd  Morgan  to  distinguish  between  the 
human  method  of  solving  a  problem  and  the  dog's  method, 
the  latter  being  called  "trial  and  error"  (282,  p.  139).  Mor- 
gan meant  that  the  dog  does  not  attempt  to  reason  the  matter 
out  beforehand,  making  use  of  his  previously  acquired  knowl- 
edge before  beginning  to  act ;  but  that  he  attacks  it  at  once 
in  some  manner  derived  from  individual  experience  or  racial 
inheritance.  If  this  method  fails,  he  tries  another  similarly 
derived,  and  so  on  until  one  method  proves  successful. 
Paramecium  also  tries  over  and  over  again,  although  what 
it  tries  is  always  the  same  thing.  Whether  Paramecium's 
behavior  is  really  shown  to  be  akin  to  the  dog's,  by  calling 
both  "trial  and  error,"  is  questionable,  however ;  the  resem- 
blances between  the  performances  of  an  animal  that  invariably^ 
responds  with  the  same  reaction  until  it  chances  to  be  carried 
beyond  the  reach  of  the  stimulus,  and  those  of  a  human  being 
who  successively  "thinks  of,"  that  is,  recalls  the  ideas  of 


Modification  by  Experience  207 

various  devices  until  the  right  one  is  obtained,  is  but  super- 
ficial. Certainly  the  behavior  of  the  Paramecium,  "trial 
and  error"  though  it  may  be,  is  not  learning,  and  gives  no 
evidence  either  for  or  against  consciousness  as  its  accompani- 
ment. If  it  has  a  subjective  side,  the  unpleasantness  that 
is  most  naturally  regarded  as  the  accompaniment  of  a  negative 
reaction  would  seem  to  be  modified  in  no  way  by  the  repeated 
performance  of  the  reaction. 

§  81.   Heightened,  Reaction  as  the  Result  of  Previous 

Stimulation 

/ 

But  even  in  the  lowest  animals  the  effect  of  a  stimulus  is 
often,  as  we  have  seen,  altered  by  the  "physiological  con- 
dition" of  the  animal,  and  this  condition  is  commonly  the 
result  of  the  stimulation  previously  received.  Sometimes  the 
influence  is  in  the  direction  of  increasing  the  violence  of  the 
response.  Thus  in  the  earthworm  Jennings  points  out  that 
various  stages  of  excitability  may  exist,  due  to  the  action  of 
previous  stimulation  and  varying  all  the  way  from  a  state  of 
rest,  where  a  slight  stimulus  produces  no  effect,  to  a  condi- 
tion of  violent  excitement,  where  moderate  stimulation  will 
cause  the  animal  to  "whip  around"  into  a  reversed  position 
or  wave  its  head  frantically  in  the  air  (210).  This  increased 
excitability  suggests  the  "nervous  irritation"  produced  in  a 
human  being  by  an  accumulation  of  disagreeable  stimuli; 
but  an  increased  unpleasantness  is  the  only  obvious  interpre- 
tation of  its  psychic  aspect. 

§  82.    Cessation  of  Reaction  to  a  Repeated  Stimulus 

While  response  to  a  given  stimulus  may  thus  be  altered  by 
reason  of  the  fact  that  other  stimuli  have  been  acting  upon 
the  animal  just  previously,  certain  interesting  modifications  of 
reaction  occur  when  the  same  stimulus  is  repeatedly  given. 


208  The  Animal  Mind 

One  form  of  such  modification  is  found  where  the  stimulus 
is  of  moderate  intensity  and  not  harmful  to  the  animal. 
The  Ciliata  Vorticella  and  Stentor,  which  spend  a  part  of  their 
time  attached  to  solids  by  a  contractile  stem,  contract  at  the 
first  application  of  a  moderately  intense  mechanical  stimulus, 
but  fail  to  react  at  all  when  the  stimulus  is  several  times 
repeated  (203).  Hydra  responds  to  mechanical  stimulation 
by  contraction,  but  gets  used  to  the  process  when  repeated 
and  gives  no  further  reaction  (418).  The  sea-anemone 
Aiptasia  reacts  by  a  sharp  contraction  to  a  drop  of  water 
falling  on  it ;  later  it  ceases  its  response  to  this  stimulus.  If 
exposed  to  light,  it  contracts  and  remains  in  this  state  for  some 
hours,  but  afterwards  expands  again  (207).  The  annelid 
Bispira  voluticornis  was  found  by  Hesse  to  give  no  further 
response  to  sudden  shadows  when  the  stimulus  was  frequently 
repeated  (173).  Von  Uexkiill  reports  that  the  sea  urchin 
Centrostephanus  longispinus  ceased  to  respond  to  shadows 
after  three  successive  stimulations  (410).  Nagel  observed 
that  certain  eyeless  mollusks  which  react  to  sudden  darkening 
very  quickly  get  used  to  the  stimulus  and  cease  to  respond; 
often  after  one  reaction  they  decline  to  react  for  several  hours.1 
The  mollusks  that  responded  to  sudden  brightening  rather 
than  to  shadows,  that  were  in  NageFs  phrase  photoptic  rather 
than  skioptic,  took  longer  to  become  accustomed  to  repeated 
stimulation,  but  did  so  by  gradually  weakening  their  reaction 
(290).  A  web-making  spider  that  was  found  by  the  Peck- 
hams  to  drop  from  its  web  at  the  sound  of  a  large  tuning 
fork  declined  to  disturb  itself  after  the  stimulus  had  been 

1  The  opposite  phenomenon  is  reported  by  Rawitz  of  the  mollusk  Pecten, 
whose  response  to  a  shadow  was  the  shutting  of  its  shell.  Repeated  or  long- 
continued  shadowing,  instead  of  doing  away  with  the  reaction,  caused  the 
animal  to  remain  with  closed  shell  for  a  long  time ;  an  intensification  of  the 
reaction  which  suggests  the  effect  of  summation  of  stimuli  (360).  We  may 
infer  that  the  stimulus  in  such  a  case  is  injurious. 


Modification  by  Experience  209 

repeated  from  five  to  seven  times  (320).  Ants  "become 
used"  to  the  ultra-violet  rays  which  they  ordinarily  avoid 
(ng). 

If  learning  by  experience  be  extended  to  cover  every  case 
where  an  animal  reacts  to  a  stimulus  differently  because  of 
earlier  stimulation,  then  this  is  learning  by  experience.  An 
interesting  point  suggests  itself  in  regard  to  the  permanency 
of  such  learning.  In  case  the  animal  the  next  day  responds 
with  less  vigor  to  the  excitant  which  it  got  used  to  the  day 
before,  there  would  seem  some  plausibility  about  the  inter- 
pretation of  Nagel,  who  says  with  that  inclination  in  favor 
of  the  psychic  which  always  characterizes  him,  that  the  be- 
havior of  his  mollusks  "makes  the  assumption  of  a  certain 
power  of  judgment  in  these  animals  unavoidable.  The 
animal  recognizes  that  the  repeated  shadow  is  not  due  to  the 
presence  of  an  enemy  or  other  danger"  (290).  On  the  other 
hand,  of  course,  it  is  perfectly  conceivable  that  an  animal 
might  go  through  such  a  process  of  judgment  and  still  be 
unable  to  remember  it  the  next  day.  However,  if  we  find 
that  only  very  recent  stimulation  has  any  effect,  the  suggestion 
that  this  effect  is  due  to  some  purely  physiological  alteration 
in  the  organism  lies  near  at  hand. 

As  a  matter  of  fact,  the  higher  the  animal  the  more  lasting 
appears  to  be  the  result  of  "getting  used"  to  a  stimulus. 
For  instance  Hydra,  if  it  is  allowed  to  reach  full  expansion 
after  having  contracted  at  a  touch,  will  respond  to  the 
seco'nd  touch  just  as  it  did  to  the  first ;  the  stimuli,  to  influence 
each  other,  must  come  in  quick  succession.  The  relation 
of  loss  of  reactive  power  to  the  interval  between  the  stimuli 
was  prettily  shown  by  Hargitt  for  a  tube-dwelling  marine 
worm,  Hydroides  dianthus.  Shadows  were  thrown  from  a 
pendulum  whose  rate  could  be  varied,  and  it  was  found  that 
if  a  full  second  intervened  between  the  stimuli,  the  reaction 
p 


2io  The  Animal  Mind 

would  always  be  given ;  if  the  interval  was  half  a  second,  after 
the  first  few  stimuli  many  of  the  worms  failed  to  react,  while 
if  the  interval  was  only  a  quarter  of  a  second,  almost  all  of 
them  became  indifferent  (158).  Mrs.  Yerkes  observed  that 
the  same  annelid  would  often  fail  to  respond  to  shadows 
repeated  at  intervals  of  from  5  to  10  seconds,  and  that  95 
out  of  200  responded  when  the  interval  was  from  one  to  two 
minutes  (447).  On  the  other  hand,  the  spider  experimented 
on  by  the  Peckhams  for  some  time  reacted  each  day  to  the 
sound  of  the  fork  by  dropping  from  its  web  until  the  sound 
had  been  repeated  some  half  dozen  times;  but  after  the 
fifteenth  day  it  would  not  drop  at  all  (320).  There  is  an 
adaptive  aspect  to  this  difference  between  Hydroides  and  the 
spider.  An  animal  that  has  little  power  to  discriminate 
among  stimuli  could  not  afford  to  suspend  its  negative  re- 
action for  any  length  of  time,  for  another  stimulus,  indis- 
tinguishable from  the  one  to  which  it  had  become  accustomed, 
might  happen  along  and  end  its  career.  But  a  creature  with 
greater  capacity  for  qualitative  discrimination  can  safely 
suspend  reaction  for  a  considerable  period  to  one  out  of  the 
many  stimuli  which  it  is  capable  of  discriminating. 

Where  the  effect  is  temporary,  the  most  obvious  suggestion 
as  to  its  cause  is  fatigue.  In  our  own  experience  this  word  is 
used  chiefly  with  reference  to  motor  processes;  we  perceive 
a  certain  signal,  but  are  too  fatigued  to  respond.  On  the 
sensory  side,  when  a  repeated  stimulus  is  no  longer  perceived, 
we  call  the  phenomenon  one  of  adaptation.  That  the  failure 
of  Stentor  to  respond  to  successive  stimuli  is  not  due  to  motor 
fatigue  appears  quite  certain  to  Jennings,  since  under  favor- 
able conditions  he  has  obtained  reactions  from  the  animal 
for  a  far  longer  period  than  that  occupied  by  the  process  of 
getting  used  to  slight  mechanical  stimulation  (203).  And  in 
most  of  the  cases  cited,  the  acclimatizing  process  seems  to 


Modification  by  Experience 


211 


occur  too  rapidly  to  make  fatigue  of  the  motor  apparatus 
probable.  The  most  natural  analogy  to  the  phenomenon  in 
our  own  experience  is  sensory  adaptation,  such  as  we  find,  for 
instance,  in  the  fact  that  a  moderate  weight  laid  on  the  skin 
ceases  after  a  time  to  be  felt.  The  psychic  accompaniment 
of  such  modification  of  behavior  is  probably,  if  it  exists, 
merely  the  gradual  disappearance  of  all  sensation. 

Another  case  of  the  cessation  of  reaction  to  a  repeated  stim- 
ulus is  reported  by  Wasmann  of  ants  in  an  artificial  nest, 
which  assumed  the  fighting  attitude  in  response  to  the  move- 
ment of  a  finger  outside  the  nest,  but  after  two  or  three  repeti- 
tions of  the  motion  were  no  longer  disturbed  (426).  Where 
animals  as  high  in  the  scale  as  the  ant  and  spider  are  con- 
cerned, it  is  possible  that  this  process  of  getting  used  to  a 
stimulus  may  involve  rather  a  dulling  of  emotion  than  a  dis- 
appearance of  sensation. 

That  adaptation  is  itself  adaptive  hardly  needs  to  be  empha- 
sized. As  Jennings  suggests,  if  the  sea-anemone  that  con- 
tracts at  the  first  ray  of  light  were  to  remain  contracted  in 
steady  illumination,  it  would  lose  all  chance  of  getting  food 
under  the  new  conditions  (207).  The  negative  reactions 
ordinarily  involve  interruption  of  the  food-taking  process, 
and  it  is  important  that  they  should  not  be  continued  in  re- 
sponse to  stimulation  that  is  relatively  permanent.  Hargitt 
thinks  that  the  loss  of  reaction  to  repeated  shadows  which  he 
observed  in  marine  worms  may  be  an  adaptation  to  the  vary- 
ing illumination  caused  by  ripples  at  the  surface  of  the  water 

(158). 

Does  such  loss  of  reactive  power  ever  occur  in  connection 
with  a  positive  or  food-taking  reaction?  One  would  expect 
that  a  single  condition  would  bring  it  about  under  such  cir- 
cumstances ;  namely,  loss  of  hunger.  And,  as  a  matter  of 
fact,  observers  of  the  feeding  processes  in  many  lower  forms 


212  The  Animal  Mind 

have  found  that  these  cease  or  turn  into  negative  responses 
when  the  animal  is  satiated ;  although  Pieron  indeed  reports 
that  while  the  responses  of  Actinia  equina  and  A.  rubra  to 
mechanical  stimulation  cease  on  repetition  of  the  stimulus, 
those  to  food  stimulation  continue  indefinitely  (327).  If  the 
change  from  food-taking  to  negative  reaction  has  a  conscious 
accompaniment,  this  might  naturally  be  thought  of  as  a 
change  from  pleasant  to  unpleasant  affective  tone.  One 
very  interesting  case  of  such  a  change  in  the  feeding  reaction 
occurs  in  the  sea-anemone.  Nagel  observed  that  if  a  ball  of 
filter  paper  soaked  in  fish  juice  were  placed  upon  one  of  the 
tentacles  of  Adamsia,  it  was  seized  as  eagerly  as  a  ball  of  fish 
meat,  but  that  when  this  deception  had  been  several  times 
repeated,  the  ball  was  held  for  a  shorter  period  each  time,  and 
was  finally  rejected  as  soon  as  offered.  Nagel  is  inclined  to 
think  that  this  is  learning  by  experience,  and  points  out  that 
the  psychic  life  of  Adamsia  must  possess  little  unity,  for  the 
"experience"  of  one  tentacle  does  not  lead  other  tentacles  to 
reject  the  paper  balls  at  once  (291).  Parker  finds  similar 
behavior  in  Metridium,  and  explains  it  by  saying  that  the 
filter  paper  offers  but  a  weak  food  stimulus,  and  that  "  the  suc- 
cessive application  of  a  very  weak  stimulus  is  accompanied  by 
...  a  gradual  decline  in  the  effects,  till  finally  the  response 
fails  entirely" ;  in  other  words,  that  we  have  adaptation  to  a 
food  stimulus  (303).  Jennings  fed  Aiptasia  alternately  with 
pieces  of  crab  meat  and  with  filter  paper  soaked  with  meat 
juice,  the  result  being  that  the  fifth  piece  of  filter  paper  was 
rejected  —  but  so  was  the  crab  meat  thereafter.  Jennings 
came  to  the  conclusion  that  the  phenomenon  is  due  simply 
to  loss  of  hunger  on  the  animal's  part,  and  that  where  Parker 
found  that  the  crab  meat  would  be  taken  after  the  filter  paper 
was  refused,  it  was  because  the  latter  was  a  weaker  stimulus 
and  naturally  was  the  first  to  call  forth  the  effects  of 


Modification  by  Experience  213 

satiety.  The  objection  to  the  hunger  hypothesis  is  that 
other  tentacles  of  the  same  animal  will  react  after  one 
tentacle  has  stopped;  satiety  ought  surely  to  affect  the 
entire  organism  (207).  Allabach,  in  the  light  of  these 
researches,  made  a  careful  study  of  Metridium.  She  dis- 
poses of  the  psychic  learning  by  experience  theory  of  Nagel 
by  saying  that  the  only  experience  upon  which  the  animal 
could  reject  the  filter  paper  must  be  experience  that  it  is  not 
good  for  food.  This  could  be  learned  only  by  swallowing  it ; 
but  the  failure  of  the  reaction  occurs  just  as  well  when  the 
animal  is  prevented  from  swallowing  the  filter  paper.  That 
the  phenomenon  is  not  one  of  adaptation  to  weak  stimuli 
is  shown  by  the  fact  that  it  may  be  brought  about  by  succes- 
sive feedings  with  meat  which  is  not  allowed  to  be  swallowed. 
It  cannot  be  due  to  loss  of  hunger,  for  this  is  experimentally 
shown  to  affect  all  the  tentacles  at  once.  Allabach  concludes 
that  it  is  simply  a  case  of  local  fatigue  of  the  tentacles.  The 
taking  of  food  by  a  tentacle  involves  the  production  of  a  con- 
siderable quantity  of  mucus,  the  immediate  supply  of  which 
is  probably  exhausted  after  a  few  reactions,  and  a  short  period 
of  rest  is  required  (3). 

This  explanation  seems,  however,  not  precisely  adapted  to 
the  most  recently  published  experimental  results  bearing  upon 
the  point ;  those  of  Fleure  and  Walton.  They  tested  Actinia 
with  a  scrap  of  filter  paper  once  every  twenty-four  hours, 
placing  it  on  the  same  tentacles,  which  usually  carried  it  to 
the  mouth,  where  it  was  swallowed  and  later  ejected.  After 
from  two  to  five  days  the  mouth  would  no  longer  swallow  the 
fragment,  and  in  two  more  days  the  tentacles  refused  to  take 
hold  of  it.  Other  tentacles  could  be  "deceived"  at  least 
once  or  twice  after  this,  but  very  soon  manifested  the  inhibi- 
tion, indicating  that  a  nervous  connection  and  not  merely 
local  fatigue  was  involved.  All  traces  of  the  "learning" 


214  The  Animal  Mind 

were  lost  after  from  six  to  ten  days'  interval.     Another  anem- 
/    one,  Tealia,  "learned"  more  quickly  than  Actinia  (127). 

Modification  of  behavior  closely  analogous  to  this  was 
observed  in  fishes  by  Herrick.  Catfish,  when  the  barbels 
were  touched  with  a  bit  of  meat,  immediately  seized  it.  If 
a  piece  of  cotton  wool  were  used  instead  of  the  meat,  they 
made  the  same  reaction,  but  after  this  experience  had  been 
repeated  a  certain  number  of  times  they  ceased  to  respond 
to  the  cotton,  although  they  still  took  meat  eagerly,  showing 
that  neither  hunger  nor  fatigue  was  involved.  Moreover, 
the  "learning"  would  persist  for  a  day  or  two.  "I  rarely," 
says  Herrick,  "  after  the  first  trials,  got  a  prompt  '  gustatory ' 
reflex  with  the  cotton"  (165).  In  these  cases  it  looks  very 
much  as  though  we  had  to  deal  with  a  real  discrimination 
between  stimuli,  a  type  of  behavior  which  will  be  considered 
under  a  later  heading. 

§  83.    Varied  Negative  Reactions  to  a  Repeated  Stimulus 

Another  way  in  which  reactlfc^  a  ^petition  of  the  same 
stimulus  becomes  modified  is  as^^^vs :  the  animal  under  a  '\ 
strong  stimulus  tries ,   one  after  c^^^g)  different  forms  of  t 
negative  reaction  until  one  of  them  is  successful  in  getting  rid  * 
of  the  stimulus.    Here  is  a  genuine  case  of  trial  and  error, 
where,  however,  different  reactions  are  tried.     The  Stentor 
furnishes  us  with  a  typical  example  of  this :  when  attached  by 
its  stem  and  stimulated  strongly  a  number  of  times  in  suc- 
cession, it  first  tries  the  ordinary  negative  reaction,  bending 
over  and  to  one  side.     Next,  it  reverses  momentarily  the 
direction  in  which  its  cilia  are  whirling.     If  this,  several 
times  repeated,  does  not  succeed  in  getting  rid  of  the  stimulus, 
the  animal  contracts  strongly  upon  its  stem.     This  also  is 
continued  for  some  time,  but  if  the  stimulus  is  kept  up,  too, 
the  Stentor  finally  breaks  away  and  swims  off  (203). 


Modification  by  Experience  215 

There  are  many  examples  of  similar  behavior  in  other 
animals.  Hydra,  which  sometimes  displays  the  phenome- 
non of  adaptation  by  refusing  to  react  at  all  to  repeated 
stimulation,  in  other  cases  tries  first  the  ordinary  negative 
response  of  contraction,  and  later  moves  away  from  the  region 
it  has  been  occupying  (418).  Frandsen  found  that  if  the  slug 
Limax  maximus  has  a  tentacle  touched  several  times  in  suc- 
cession, it  at  first  withdraws  the  tentacle  and  turns  away  from 
the  stimulus.  Later,  it  may  move  toward  and  push  against 
the  stimulus,  and  do  the  same  if  the  touch  is  on  the  side  of 
its  body,  resisting  and  curving  around  the  obstacle  —  another 
way,  of  course,  of  getting  rid  of  it  (135).  Preyer,  again, 
observed  a  very  pretty  instance  of  this  sort  of  behavior  in  the 
starfish.  He  slipped  a  piece  of  rubber  tubing  over  the  middle 
part  of  one  of  the  arms  of  a  starfish  belonging  to  a  species  in 
which  those  members  are  very  slender,  and  found  that  the 
animal  tried  successively  various  devices  to  get  rid  of  the 
foreign  body,  to  wit,  the  following:  rubbing  it  off  against 
the  ground,  shaking  it  off  by  holding  the  arm  aloft  and  wav- 
ing it  pendulum-wise  in  the  air,  holding  the  tube  against  the 
ground  with  a  neighboring  arm  and  pulling  the  afflicted  arm 
out,  pressing  other  arms  against  the  tube  and  pushing  it  off, 
and,  finally,  as  a  last  resort,  amputating  the  arm.  This,  says 
Preyer,  is  intelligence,  for  the  emergency  is  not  one  normal  to 
the  animal,  and  it  is  adapting  itself  to  new  conditions  (350). 
It  would,  however,  be  demanding  too  much  even  from  intel- 
ligence to  suppose  that  the  starfish's  behavior  is  entirely 
new.  A  human  being,  capable  of  ideas,  could  only,  in  a 
similar  predicament,  "think  of,"  that  is,  call  up,  ideas  of  the 
behavior  which  on  former  occasions  somewhat  resembling 
the  present  had  proved  effective.  Do  such  cases  of  the  trial 
of  different  devices  indicate  that  the  animal  concerned  calls 
up  any  kind  of  idea  or  image  of  each  device,  before  putting 


216  The  Animal  Mind 

it  into  practice?  Decided  evidence  in  favor  of  such  a  sup- 
position might  be  furnished  if  the  " trial  and  error"  needed 
to  be  gone  through  with  only  once.  A  human  being  brought 
into  such  conditions  and  guiding  his  conduct  by  ideas  would, 
if  placed  in  a  similar  emergency  soon  afterwards,  immediately 
recall  the  idea  of  the  successful  action  and  waste  no  time  over 
the  unsuccessful  ones.  But  we  have  no  reason  to  think  that 
such  is  the  fact  with  our  primitive  animals.  Preyer's  star- 
fish, when  confined  by  large  flat-headed  pins  driven  into  the 
board  on  which  it  lay,  close  up  in  the  angles  between  its 
arms,  managed  to  escape  by  trying  a  large  variety  of  move- 
ments, and  gradually  diminished,  Preyer  says,  the  number  of 
useless  movements  made  in  successive  experiments  (350). 
O.  C.  Glaser,  on  the  other  hand,  has  recently  found  that  the 
echinoderm  Ophiura  brevispina  does  not  improve  at  all  with 
practice  in  removing  obstructions  from  its  arms.  The  very 
versatility  of  the  starfish,  this  writer  thinks,  tells  against 
its  perfecting  any  one  movement  through  experience  (145). 
Stentor  and  Hydra  go  through  the  same  series  of  reactions 
each  time,  without  apparently  being  influenced  by  their 
previous  behavior.  And  again  we  must  remind  ourselves 
that  there  is  no  reason  why  their  conduct,  adaptively  regarded, 
should  be  otherwise.  An  animal  with  so  little  power  of  dis- 
tinguishing qualitative  differences  among  stimuli  cannot  be 
in  any  way  aware  that  the  stimulus  which  affects  it  a  second 
time  is  going,  as  in  the  previous  case,  to  be  so  persistent  that 
the  ordinary  negative  reaction  will  not  get  rid  of  it.  Further, 
each  reaction  of  the  series  performed  by  the  animal  is  more 
disturbing  to  its  ordinary  course  of  life  than  the  preceding  one. 
The  Stentor  can  bend  to  one  side  and  still  continue  the  food- 
taking  process ;  if  it  reverses  its  ciliary  action,  feeding  must  be 
momentarily  interrupted ;  while  contraction  on  the  stem  and 
breaking  loose  from  its  moorings  are  still  more  serious  in- 


Modification  by  Experience  217 

fractions  of  the  normal  routine.  It  would  be  decidedly 
disadvantageous  to  take  the  last  step  while  there  was  any 
chance  that  milder  measures  might  prevail. 

In  all  probability,  since  the  behavior  just  described  has  no 
permanent  effect  upon  the  animal,  it  is  physiologically  due,  as 
Jennings  suggests  (208),  to  the  overflow  of  the  nervous  energy 
set  free  by  the  stimulus  into  first  one  channel  and  then  an- 
other. In  most  cases  the  movements  resulting  are  all  adapted 
to  getting  rid  of  the  stimulus,  though  only  one  of  them  is 
successful  in  so  doing ;  but  we  have  on  record  one  case  where, 
in  a  supreme  emergency,  the  stimulus  being  not  only  repeated 
but  increased  in  intensity,  every  possible  outlet  is  tried, 
whether  it  has  any  fitness  to  the  situation  or  not.  This  was 
observed  by  Mast,  testing  the  effect  of  increased  temperature 
on  the  reactions  of  planarians.  The  first  influence  of  such 
increase  from  23  degrees  to  26  degrees  C.,  is  to  produce 
heightened  activity  and  positive  reactions.  Then,  from  26 
degrees  to  38  degrees,  the  reactions  are  negative.  From  38 
degrees  to  39  degrees,  violent  crawling  movements  set  in,  and 
then,  curiously  enough,  the  righting  reaction  is  given,  per- 
fectly irrelevant,  of  course,  to  the  conditions.  Finally,  the 
anterior  and  posterior  ends  are  turned  under,  the  central  part 
is  arched  upward,  and  the  animal  falls  over  forward  on  its 
back  (260). 

In  all  these  cases  where  repetition  of  the  same  stimulus 
produces  successively  different  forms  of  the  negative  reaction 
increasing  in  violence,  it  is  most  natural  to  think  of  the  psychic 
accompaniment  as  an  increasing  degree  of  unpleasantness. 
In  our  own  experience,  repeating  a  stimulus  does  not  alter 
the  quality  of  the  resulting  sensation,  except  where  the  struc- 
ture of  a  special  sense  organ  is  a  modifying  factor,  as  in  the 
case  of  visual  after-images.  Repetition  of  the  stimulus  does 
with  us  human  beings  diminish  the  intensity  of  the  accompany- 


218  The  Animal  Mind 

ing  sensation;  but  this  process  is  the  natural  accompani- 
ment, as  we  have  seen,  of  diminishing  reaction,  not  of  varied 
and  increasingly  violent  reaction.  A  decidedly  disagreeable 
stimulus  acting  repeatedly  on  a  human  being  may  produce 
unpleasantness  that  grows  more  and  more  intense  until  it 
is  unbearable ;  the  behavior  of  a  human  being  under  such  cir- 
cumstances is  much  like  the  animal  behavior  we  have  just 
been  describing.  Various  movements  calculated  to  get  rid 
of  the  stimulus  are  tried,  each  more  energetic  than  the  last. 
Hence,  if  the  lower  animals  behaving  thus  are  conscious,  we 
may  plausibly  assert  that  their  consciousness  under  these 
circumstances  is  increasingly  unpleasant.  But  the  human 
experience  in  such  a  case  would  be,  or  might  be,  further  char- 
acterized by  the  presence  of  ideas.  That  is,  the  human  being 
would  think  of  the  different  ways  to  get  rid  of  the  stimulus 
one  after  another.  This  many,  at  least,  of  the  animals  that 
try  different  negative  reactions  are  apparently  incapable  of 
doing.  We  judge  that  they  are  so  by  the  simple  fact  that  on 
being  subjected  after  an  interval  to  the  same  presumably  dis- 
agreeable stimulus,  they  do  not  at  once  make  the  reaction  that 
was  previously  successful  in  getting  rid  of  it.  A  human  being, 
recalling  that  reaction  in  idea,  would  be  able  to  do  so.  We 
shall  see  in  the  next  chapter  that  many  animals,  while  they 
do  not  learn  the  successful  reaction  from  a  single  experience, 
do  gradually  diminish  the  number  of  unsuccessful  ones  made 
in  a  series  of  experiences.  It  may  be  that  as  more  experimen- 
tal evidence  is  accumulated,  this  will  be  found  to  be  the  case 
throughout  the  whole  animal  kingdom,  but  at  present  it 
looks  as  though  the  lowest  forms  may,  when  an  injurious 
stimulus  is  repeatedly  given,  pass  through  their  whole  reper- 
toire of  negative  reactions  in  one  experience  after  another, 
without  any  shortening  of  the  process.  Trial  and  error  this 
may  be  called :  learning  it  is  not. 


Modification  by  Experience  219 

§  84.  Dropping  off  Useless  Movements:  the  Labyrinth 
Method 

The  next  form  of  modification  of  behavior  by  individual 
experience  which  we  shall  consider  occurs  when  an  animal, 
under  the  influence  of  some  stimulus  which  it  strives  either 
to  get  rid  of  or  to  get  more  of,  goes  through  a  series  of  reactions 
until  one  proves  successful ;  on  being  after  an  interval  of  time 
placed  in  the  same  situation,  the  unsuccessful  movements  are 
fewer,  and  further  repetition  causes  them  to  be  dropped  off  en- 
tirely. This  is  the  mode  of  behavior  which  was  first  brought 
into  clear  relief  by  the  experiments  of  Thorndike  on  chicks, 
dogs,  and  cats.  Since  then  an  increasing  number  of  inves- 
tigators have  shown  its  existence  in  a  large  number  of  forms. 
One  of  the  simplest  methods  for  the  testing  of  this  sort  of  learn- 
ing is  the  labyrinth  method.  In  its  developed  form  it  was  first 
used,  I  believe,  by  Small  in  his  work  on  white  rats,  and  was 
suggested  by  the  natural  habits  of  the  animal,  which  is,  of 
course,  accustomed  to  run  about  through  narrow  passages. 
The  plan  consists  in  placing  food,  or  something  else  attractive 
to  the  animal,  at  the  end  of  a  series  of  passages  containing 
a  number  of  false  turns.  The  labyrinth  used  for  the  rats 
was  very  complicated,  being  in  fact  a  replica  in  wire  netting 
of  the  Hampton  Court  maze,  but  much  simpler  ones  have 
since  been  employed  for  other  animals.  One  advantage  of 
the  labyrinth  method  is  that  it  requires  nothing  of  the  animal 
except  what  is  perfectly  natural  to  it,  namely,  locomotion. 

The  lowest  forms  which  have  been  thus  far  tested  by  this 
means  are  certain  Crustacea.  The  crab  Carcinus  granulatus 
was  placed  in  a  very  simple  labyrinth  with  only  two  points 
where  a  choice  between  the  right  and  wrong  paths  was  pos- 
sible. At  the  end  of  the  labyrinth  was  the  aquarium,  and  the 
crab's  discomfort  out  of  the  water  served  as  the  occasion  stir- 


220 


The  Animal  Mind 


G 


ring  it  to  activity.  In  fifty  experiments  the  path  had  not  been 
perfectly  learned,  although  the  time  was  greatly  reduced.  A 
still  simpler  path  was  then  offered  the  animal  by  placing  a 
wire  screen  partition  in  the  middle  of  the  aquarium,  with  an 
opening  in  the  centre  and  food  on  the  other  side ;  and  in  ten 
trials  the  time  occupied  by  the  crab  in  finding  the  food  was 
much  lessened.  Still  neither  of  the  two  animals  tested  had 
learned  to  go  straight  to  the  opening,  but  each  followed  a 
habit  of  its  own,  one  moving  directly  toward  the  food,  hunt- 
ing for  an  opening 
near  it,  and  then 
going  to  the  mid- 
dle where  the 
opening  was;  the 
other  always  fol- 
lowing the  edge  of 
the  screen  all  the 
way  around  until 
it  came  upon  the 

FIG.  13.  —  Labynnth  used  by  Yerkes  and  Huggms  in  ex-  .  r 

periments  on  the  crayfish.      T,  compartment  from  Opening    ^453/* 
which  animal  was  started;    P,  partition  at  exit;      A  labyrinth  off 6 r- 

G,  glass  plate  closing  one  exit.  .  . 

ing  only  a  single 

choice  of  passages  was  used  in  testing  the  crayfish ;  again  one 
end  of  the  box  communicated  with  the  aquarium  (Fig.  13). 
About  halfway  down  the  length  of  the  box  a  partition  put  in 
longitudinally  divided  it  into  two  passages,  one  of  which  was 
closed  at  the  end  by  a  glass  plate.  In  sixty  trials  the  animals, 
which  had  originally  chosen  the  correct  passage  50  per  cent 
of  the  time,  came  to  choose  it  90  per  cent  of  the  time.  A 
second  series,  with  a  single  animal  upon  which  more  tests  a 
day  were  made,  resulted  in  the  formation  of  a  perfect  habit  in 
two  hundred  and  fifty  experiments.  The  glass  plate  was  then 
shifted  to  the  other  passage,  and  the  crayfish  was  naturally 


Modification  by  Experience 


221 


completely  baffled  for  a  time,  but  succeeded  in  learning  the 
new  habit  (47 1 ).  Ants  of  the  species  Stenammafulvum  piceum 
have  been  tested  in  a  labyrinth  by  Fielde.  The  observations 
indicated  that  this  ant's  tendency  to  be  guided  by  the  chemical 
traces  of  its  own  footsteps  militated  to  a  certain  extent  against 
shortening  the  path  by  dropping  off  useless  turnings.  Each 
ant  followed  her  own  previous  trail  through  the  labyrinth  to 
the  nest.  Yet  some  tendency  for  the  movements  to  become 


FIG.  14.  —  Labyrinth  used  by  Yerkes  in  experiments  on  the  frog.  A,  box  open- 
ing into  maze ;  E,  entrance ;  T,  tank ;  G,  glass  plate ;  P,  partition ;  7C, 
electric  circuit  whereby  animal  could  be  given  shock  on  entering  wrong 
passage;  C,  K,  cells  and  key;  R,  R,  red  cardboard;  W,  W,  white  card- 
board. 

automatic  and  independent  of  the  smell  clew  was  shown  by 
the  fact  that  when  an  ant  had  gone  over  the  path  many  times, 
a  portion  of  the  track  might  be  obliterated  without  inter- 
rupting her  course  1  (118). 

A  simple  form  of  the  labyrinth  method  has  been  used  on 
fish  (Fundulus),  which  were  kept  by  a  screen  in  the  sunny  end 
of  an  aquarium,  the  darkened  end  being  also  the  place  where 
they  were  fed.  One  upper  corner  of  the  screen  was  cut  out. 
In  a  couple  of  days,  allowing  six  or  eight  trials  a  day,  the  fish 
learned  to  swim  straight  up  to  this  corner  (394).  A  more 
complicated  labyrinth  was  also  used,  but  the  time  required 

1  Cf.  the  observations  of  Pieron  referred  to  on  p.  94. 


222 


The  Animal  Mind 


to  learn  it  is  not  stated.  The  greater  speed  and  ease  of 
locomotion  in  the  fish  as  compared  with  the  crustaceans 
may  have  been  one  factor  concerned  in  the  former's  greater 
rapidity  of  learning. 

With  the  green  frog,  the  labyrinth  pictured  in  Figure  14 
was  used.  After  one  hundred  trials,  practically  no  errors  were 
made  (454).  Another  animal  whose  learning  powers  have 

been  tested  by   this 


method  is  the  turtle. 

The  labyrinth  was 
distinctly  more  com- 
plex than  that  used 
for  the  frog.  It  in- 
volved four  blind 
passages,  and  led  to 
the  turtle's  comfort- 
able, darkened  nest. 

During  the  first 
four  trips  the  time 
was  reduced  from 

FIG.  15.  —  Labyrinth  used  by  Yerkes  with  turtles.     ,,  .  J      r 

A,  starting  point;    F,   blind   alley;    3,   4,   6,     thirty-five  minutes  to 

inclined  planes.  three     minutes    and 

thirty  seconds;  in  the  fourth  trip  the  animal  took  two 
wrong  turns.  The  time  of  the  fiftieth  trip  was  thirty-five 
seconds.  In  a  second  labyrinth  (Fig.  15),  two  inclined 
planes  were  introduced,  up  and  down  which  the  turtles  had 
to  crawl.  This  labyrinth  took  them  longer  to  traverse,  and 
the  time  curve  shows  greater  irregularity,  rising,  for  instance, 
to  seven  minutes  on  the  forty-fifth  trial,  after  having  been  as 
low  as  two  minutes  and  forty-five  seconds  at  the  thirty-fifth. 
The  process  of  shortening  the  path  was  observed  very  prettily 
in  connection  with  the  inclined  planes.  The  turtles  had  to 
turn  about  as  soon  as  they  had  reached  the  bottom  of  the 


Modification  by  Experience  223 

descending  plane.  They  soon  began  to  make  the  turn  before 
they  got  to  the  bottom,  and  finally  to  throw  themselves  over 
the  edge  as  soon  as  they  reached  the  top  (450). 

Some  of  Thorndike's  experiments  on  chicks  involve  the 
labyrinth  method,  others  what  we  shall  call  the  puzzle-box 
method.  The  chicks  were  confined  in  small  pens,  with  food 
outside.  In  some  cases  they  could  get  out  by  running  to  a 
particular  spot,  or  up  an  inclined  plane;  in  other  cases  by 
pecking  or  pulling  at  something.  Both  sorts  of  action  were 
learned;  obviously  the  former,  involving  simple  locomotion 
on  the  animal's  part,  are  the  ones  which  concern  us  at  pres- 
ent (393).  Porter  found  that  the  English  sparrow  quickly 
learned  the  Hampton  Court  maze  (344),  and  that  the  vesper 
sparrow  and  cowbird  learned  a  simpler  form  in  twenty  or 
thirty  trials  (345).  Pigeons  tested  by  Rouse  acquired  the 
ability  to  traverse  four  different  labyrinths,  and  it  was  noted 
that  their  experience  with  the  earlier  ones  seemed  to  help 
them  in  the  later  ones  (371). 

White  rats  observed  by  Small  learned  the  Hampton  Court 
maze,  in  nine  experiments  made  at  intervals  of  two  days, 
so  well  that  they  committed  only  two  errors  in  the  ninth 
test,  but  the  significance  of  this  time  is  obscured  by  the 
fact  that  the  rats  were  allowed  to  run  freely  about  the  laby- 
rinth every  night  (385).  Watson's  earlier  work  with  the 
white  rat  was  designed  to  compare  the  learning  processes  of 
the  young  with  those  of  the  adult  animal.  The  rat  is  born 
unable  to  care  for  itself,  and  before  those  observed  by  Wat- 
son had  reached  the  age  of  twelve  days,  they  were  unable  to 
find  their  way  by  a  simple  labyrinth  path  back  to  the 
mother.  At  twenty-three  days  of  age  they  learned  a 
labyrinth  more  quickly  than  adults,  probably  because  of 
their  greater  activity,  although  for  the  same  reason  they 
made  more  useless  movements.  The  object  of  the  research 


224  The  Animal  Mind 

was  to  test  Flechsig's  theory  that  learning  depends  upon  the 
presence  of  medullated  fibres  in  the  central  nervous  system; 
this  was  found  to  be  unconfirmed,  since  at  twenty-four  days, 
when  the  rat  is  psychically  mature,  the  medullation  of  its 
fibres  is  highly  imperfect  (430). 

In  his  later  experiments  on  white  rats  Watson's  aim  was 
to  investigate  the  nature  of  the  sensations  which  guide  them 
through  a  labyrinth.  The  results  will  be  discussed  a  few 
pages  farther  on  (431). 

Allen's  work  on  the  guinea  pig  was  intended  for  comparison 
with  Watson's  study  of  the  white  rat,  because  the  young 
guinea  pig  comes  into  the  world,  not  helpless  like  the  baby  rat, 
but  well  equipped  on  both  the  sensory  and  motor  sides.  In  the 
labyrinth,  here,  the  mother  was  put  at  the  end  of  the  maze, 
and  the  sight  and  smell  of  her  were  supposed  to  serve  as  the 
stimulus  to  activity.  Before  the  young  animals  had  reached 
the  age  of  two  days,  they  did  not  succeed  in  learning  a  com- 
paratively simple  path,  but  at  that  age  they  did  learn  it,  and 
proved  the  fact  when  the  wire  netting  box  in  which  they  were 
placed  was  turned  about,  by  pushing  at  the  place  where  the 
opening  had  formerly  been.  At  three  days,  they  learned  a 
more  complex  labyrinth,  and  appeared  to  possess  the  learning 
capacity  of  adults  (4). 

In  Yerkes's  study  of  the  Japanese  dancing  mouse,  the 
reactions  to  irregular  and  to  regular  labyrinths  were  com- 
pared, and  it  was  found  that  a  maze  of  the  latter  type,  that  is, 
one  where  left  and  right  turns  alternated,  was  more  quickly 
learned  and  more  perfectly  mastered  than  an  irregular  one. 
Yerkes  urges  the  importance  of  keeping  account  of  the  errors 
made  by  an  animal  as  well  as  the  times  occupied  in  traversing 
a  maze  (469).  Watson's  later  work  on  the  white  rat  gives 
only  the  turns  (431).  In  many  cases,  especially  with  animals 
not  naturally  active,  the  time  values  have  little  significance; 


Modification  by  Experience  225 

an  animal  in  a  sluggish  mood  may  traverse  a  path  very 
slowly  and  yet  make  no  errors. 

Kinnaman  taught  two  Macacus  rhesus  monkeys  the  Hamp- 
ton Court  maze.  That  they  had  an  anticipatory  idea  of  the 
pleasure  in  store  for  them  at  the  centre  he  thinks  evidenced 
by  the  fact  that  they  would  begin  to  smack  their  lips  audibly 
on  reaching  the  latter  part  of  their  course.  Yet  for  them,  as 
for  the  rats,  one  of  the  most  persistent  errors  lay  in  taking  the 
wrong  turning  at  the  outset  (221). 

What  is  the  mental  aspect  of  the  process  of  learning  a  laby- 
rinth ?  Does  it  involve  that  form  of  memory  which  consists 
in  the  revival  of  images  of  past  experience  ?  Or  is  it  simply 
the  gradual  formation  of  a  habit  of  movement,  at  no  stage  of 
which  a  memory  image  functions  ?  In  the  first  place,  we  may 
note  that  no  method  less  calculated  to  involve  images  could 
well  be  devised.  A  human  being  in  such  a  labyrinth  as  that  at 
Hampton  Court,  with  all  his  wealth  of  image- forming  and 
controlling  power,  is  at  a  loss  to  make  use  of  it  for  his  guid- 
ance. Secondly,  there  are  various  phenomena  displayed  in 
the  experiments  which  tell  against  the  image  theory.  For 
one  thing,  the  slowness  of  the  learning  process  in  the  simple 
labyrinths  indicates  that  memory  in  this  sense  is  not  concerned. 
When  an  animal  has  the  choice  between  two  passages  only, 
if  it  possessed  the  power  of  recalling,  in  any  terms  whatever, 
a  memory  image  of  its  previous  experience,  surely  thirty  or 
forty  trials  would  not  be  required  before  the  right  path  was 
taken  at  once.  Again,  the  nature  of  the  errors  made  in  some 
cases  suggests  that  memory  images  are  not  present.  For  in- 
stance, when  Small's  two  rats  had  learned  the  complicated 
labyrinth  almost  perfectly,  the  one  error  in  which  they  both 
persisted  lay  in  taking  the  wrong  turn  at  the  entrance.  Now 
this,  it  is  safe  to  say,  would  be  the  very  first  error  that  a  being 
which  guided  itself  by  images  would  eliminate.  It  might  be 
Q 


226  The  Animal  Mind 

difficult  to  remember,  in  the  image- forming  sense,  the  later 
turns,  but  surely  "turn  to  the  right"  or  "turn  to  the  left" 
would  present  itself  in  some  sort  of  terms  at  the  entrance,  if 
the  animal  could  have  memory  ideas  at  all.  Furthermore,  it 
is  very  difficult  to  interpret  the  learning  process  here  as  a  case 
of  association  of  ideas.  In  Small's  labyrinth,  two  kinds  of 
errors  could  be  made:  the  one  would  land  the  animal  in  a 
cul-de-sac,  the  other  simply  meant  taking  a  longer  passage 
when  a  shorter  one  would  suffice.  If  the  former  came  to  be 
avoided  as  the  result  of  the  calling  up  of  a  memory  idea,  this 
idea  might  be  that  of  being  brought  up  short  and  compelled 
to  retrace  one's  steps,  but  how  are  we  to  imagine  the  idea 
of  a  shorter  path  as  balanced  against  that  of  a  longer  path  ? 
Small  says  they  must  be  "distance  or  temporal  ideas  in  tactual- 
motor  terms,"  and  urges  that  our  own  lack  of  experience  of 
such  ideas  should  not  make  us  doubt  their  existence  in  the 
rat  mind;  but  Thorndike's  position,  that  no  ideas  are  in- 
volved at  all,  that  the  rat  merely  comes  gradually  to  "feel 
like"  taking  one  turn  rather  than  the  other,  seems  more 
probable.1  In  other  words,  we  have  the  formation  of  a  habit 
of  movement  rather  than  an  association  of  ideas. 

But  though  ideas  may  not  be  involved,  the  further  question 
remains  as  to  what  kind  of  peripherally  excited  sensations  are 
influential  in  the  learning  process.  This  question  really 
resolves  itself  into  two.  First,  by  what  "clews"  does  the 
animal  guide  itself  in  learning  the  labyrinth  path?  Second, 
do  these  clews  continue  necessary  to  its  guidance  when  the 
habit  is  formed?  The  two  parts  of  the  problem  have  not 
always  been  kept  distinct  by  those  who  have  used  the  method. 

As  regards  the  first  part,  Small  obtained  evidence  that  his 
white  rats  were  not  guided  merely  by  the  smell  of  their  own 
tracks  in  finding  their  way  to  the  centre  of  the  maze,  from 

1  See  his  review  of  Small's  work,  Psych.  Rev.,  vol.  8,  p.  643. 


Modification  by  Experience  227 

various  observations,  among  others  the  fact  that  they  ran  all 
over  the  passages  in  their  earlier  trials,  so  that  smell  might 
have  guided  them  wrong  as  well  as  right  (385).  This  conclu- 
sion was  confirmed  by  the  experiments  of  Watson,  who  found 
that  rats  with  the  olfactory  lobes  removed  learned  the  laby- 
rinth as  readily  as  normal  rats  (431).  Yerkes,  in  his  labyrinth 
tests  upon  the  crawfish  and  the  frog,  excluded  smell  as  a  means 
of  guidance  by  washing  the  labyrinth  out  between  trials  (454, 
471).  For  certain  species  of  ants,  as  we  know,  smell  is  the 
dominant  factor.  Visual  clews  seem  to  be  used  by  different 
animals  to  different  degrees.  The  frog  displayed  a  disturb- 
ance of  its  habit  when  red  and  white  cards  placed  on  either 
side  of  the  passage  were  interchanged  (454).  The  crawfish 
seemed  to  recognize  and  draw  back  from  the  screens  in  the 
blind  passage  before  running  against  them  (471).  The  pigeons 
tested  by  Rouse,  when  required  to  go  through  the  labyrinth  in 
darkness,  were  obliged  to  relearn  it,  although  they  made  the 
first  turn  correctly.  Perhaps,  Rouse  suggests,  the  stimulus  to 
the  first  turn  was  the  sound  of  the  door  lifted  to  admit  them, 
or  the  touch  of  the  narrow  entrance  (371).  On  the  other 
hand,  Small  found  that  altering  the  direction  of  the  light  had 
little  effect  on  the  performances  of  his  white  rats.  He  also 
placed  wooden  pegs  painted  red,  at  each  division  of  the  paths, 
in  the  middle  of  the  correct  path,  and  caused  the  labyrinth 
thus  arranged  to  be  learned  by  hitherto  untrained  rats.  They 
did  not  learn  it  any  faster  through  the  presence  of  these  visual 
hints,  nor,  when  it  had  been  learned,  were  they  at  all  discom- 
posed by  the  removal  of  the  pegs  (385).  Allen's  guinea  pigs 
did  not  alter  their  behavior  with  alteration  of  the  position  of 
colored  cards  placed  as  guiding  marks  (4).  And  Watson's 
blinded  rats  learned  the  labyrinth  as  readily  as  normal 
ones  (431). 

Rouse  found  that  the  pigeon  could  make  use  of  auditory 


228  The  Animal  Mind 

stimuli  as  clews.  He  arranged  to  have  an  ordinary  electric 
bell  rung  whenever  the  birds  entered  a  wrong  alley,  and  a 
wooden  bell  sounded  when  they  emerged  and  took  the  right 
course.  After  they  had  learned  the  path  under  these  condi- 
tions, the  two  kinds  of  sound  stimuli  were  interchanged,  and 
the  result  was  a  certain  amount  of  confusion  on  the  part  of 
the  birds.  Another  device  consisted  of  a  board  with  electric 
wires,  laid  on  the  floor  of  the  labyrinth.  A  bell  was  rung 
whenever  a  pigeon  stepped  on  the  board,  and  the  bird  was 
given  an  electric  shock;  when  the  experience  had  been 
repeated  a  number  of  times,  the  pigeons  would  show  uneasi- 
ness at  the  sound  of  the  bell,  wherever  they  happened  to  be  in 
the  labyrinth  (371).  In  white  rats,  Watson  found  that  par- 
tial deafness,  produced  by  throwing  the  middle  ear  out  of 
function,  had  no  effect  on  the  ability  of  the  rats  to  learn  the 
path  (431). 

Various  stimuli  may,  then,  serve  as  clews  in  the  process  of 
learning  the  labyrinth.  But  in  certain  cases,  neither  visual, 
olfactory,  nor  auditory  stimuli  seem  to  be  at  all  concerned. 
This  was  true  of  Watson's  white  rats,  and  probably  of  Allen's 
guinea  pigs.  Special  tests  were  made  to  investigate  the  role 
of  tactile  sensations  in  the  labyrinth  performances  of  these 
animals.  With  the  guinea  pigs,  a  cardboard  labyrinth  was 
substituted  for  that  of  wire  netting,  and  a  black  cloth  placed 
on  the  floor  (4).  Watson's  white  rats,  when  the  vibrissae 
(long  whiskers)  were  removed,  learned  the  maze  as  well  as 
normal  rats.  Those  which  had  already  learned  it  manifested 
some  disturbance  on  having  the  vibrissae  removed,  showing 
a  tendency  to  bump  into  the  partitions  and  hug  the  walls. 
Watson  does  not  report  whether  this  same  disturbance  failed 
to  appear  in  the  rats  without  vibrissae  that  were  learning  the 
labyrinth  for  the  first  time ;  he  merely  gives  the  times  occu- 
pied in  traversing  the  course  to  show  that  the  learning  process 


Modification  by  Experience  229 

was  normal.  It  would  be  very  strange  and  quite  out  of  accord 
with  the  general  behavior  of  animals  in  labyrinths  if  a  prac- 
tised rat  should  be  more  disturbed  by  the  removal  of  an  accus- 
tomed stimulus  than  an  unpractised  one.  No  effect  on  the 
learning  power  of  the  rats  was  produced  by  making  their 
paws  anaesthetic  (431).  Yerkes  has  shown,  although  with- 
out operating  on  his  subjects,  that  the  Japanese  dancing 
mouse  does  not  necessarily  depend  on  sight,  smell,  or  touch 
for  guidance  in  the  labyrinth  (469).  It  should  be  noted  that 
proof  of  an  animal's  ability  to  learn  a  maze  when  deprived 
of  a  certain  class  of  sensations  does  not  show  that  it  normally 
makes  no  use  of  those  sensations  in  the  learning  process. 

As  a  matter  of  fact,  the  stimuli  which  originally  give  the 
"clews"  in  the  case  of  the  white  rats  must  be  the  rats'  own 
movements.  "Muscular  sensations  dependent  on  the  direc- 
tion of  turning,"  "  kinsesthetic  sensations,"  are  the  only 
elements  in  our  own  experience  that  suggest  themselves  as 
possibilities  where  an  animal  learns  the  maze  equally  well 
when  blinded,  anosmic,  deaf,  and  partially  deprived  of  touch 
(431).  But  this  is  not  the  same  as  saying  that  when  an  ani- 
mal has  learned  the  labyrinth,  it  is  "guided  by  kinaesthetic 
sensations."  Nor  can  we  show  that  an  animal  was  not  guided 
by  some  other  stimuli,  say  visual  ones,  in  learning  the  laby- 
rinth, when  we  prove  that  having  once  learned  it,  the  animal 
is  not  disturbed  by  the  removal  of  these  stimuli.  For  when 
the  labyrinth  path  has  been  learned,  the  habit  may  be  in  a 
sense  quite  independent  of  the  very  stimuli  that  served  to  form 
it,  precisely  as  the  pianist  becomes  independent  of  the  notes 
in  playing  a  familiar  piece.  The  fact  that  Yerkes's  frogs 
were  disturbed,  after  the  habit  had  been  formed,  by  the  inter- 
change of  the  cards,  indicates  that  visual  stimuli  were  still 
important  to  them;  but  if  they  had  not  been  disturbed  by 
such  interchange,  when  they  were  fully  practised,  it  would 


230  The  Animal  Mind 

not  have  proved  that  the  cards  had  played  no  part  in  forming 
the  habit. 

In  the  same  way,  it  is  not  likely  that  a  thoroughly  prac- 
tised animal  needs  to  have  in  consciousness  even  kinaesthetic 
sensations.  Watson  attempts  to  describe  the  processes  in 
the  mind  of  a  practised  rat  as  follows :  "  What  leads  up  to 
the  act  of  turning  ?  The  '  feeling '  (probably  only  vaguely 
'  sensed  ')  which  may  be  expressed  anthropomorphically  in 
these  terms:  *I  have  gone  so  far,  I  ought  to  be  turning 
about  now ! '  "  "  If  the  turn  is  made  at  the  proper  stage 
.  .  .  the  animal  may  be  supposed  thereby  to  get  a  *  reassur- 
ing feeling,'  which  is  exactly  comparable  from  the  stand- 
point of  control  to  the  experience  which  we  get  when  we 
touch  a  familiar  object  in  the  dark"  (431,  pp.  95-6).  I  do 
not  think  these  before  and  after  '  feelings '  are  necessarily 
present  at  all  in  the  consciousness  of  an  animal  whose 
labyrinth  habit  is  fully  formed.  Such  an  animal  has  be- 
come a  little  machine  which  takes  so  many  steps  along  a 
straight  path,  turns  to  the  right,  takes  so  many  more  steps, 
and  so  on  until  the  performance  is  complete.  If,  indeed, 
it  makes  an  error  in  this  process,  then  the  kinaesthetic  sen- 
sations may  come  into  play,  but  otherwise  there  would  seem 
to  be  no  reason  for  assuming  in  the  fully  practised  animal 
consciousness  of  any  stimulus  except  the  initial  one  which 
starts  it  on  its  path. 

Very  curious  are  the  results  obtained  by  Watson  when  the 
entire  labyrinth  was  turned  through  an  angle  of  90  degrees. 
Although  no  turn  which  the  animals  had  to  take  was  in  any 
way  altered  by  this  proceeding,  the  rats  showed  decided  con- 
fusion, the  blind  rats  as  much  as  the  others.  This  latter 
fact  would  indicate  that  alteration  in  the  direction  of  the 
light  was  not  the  source  of  the  confusion ;  but  when  the  maze 
was  rotated  through  180  degrees,  the  blind  rats  were  not 


Modification  by  Experience  231 

disturbed,  while  the  others  were.  More  investigation,  de- 
cidedly, is  needed  before  we  can  decide,  as  Watson  does, 
"  either  that  static  sensations  have  a  r61e,  or  .  .  that  the  rat 
has  some  non-human  modality  of  sensation  which,  whatever 
it  may  be,  is  thrown  out  of  gear  temporarily  by  altering  the 
customary  relations  to  the  cardinal  points  of  the  compass." 

One  or  two  incidental  observations  regarding  the  behavior 
of  animals  in  labyrinths  are  strongly  suggestive  of  the  auto- 
matic character  of  the  movements  involved.  An  animal  that 
has  gone  astray  on  the  path  will  often  find  the  way  back  to 
the  starting-point,  and  from  there  traverse  the  whole  road 
rapidly  and  unerringly  (e.g.  450,  431),  apparently  in  the  same 
way  that  a  piano  player  who  has  a  piece  "  at  his  fingers'  ends," 
but  has  stumbled  in  a  passage,  can  go  through  with  entire 
success  if  he  starts  over  again.  As  piano  players  know,  in 
such  a  case  it  is  much  better  not  to  attend  to  stimuli  at  all, 
but  to  think  of  something  else ;  the  movements  will  take  care 
of  themselves  better  if  consciousness  intervenes  as  little  as 
possible. 

Again,  in  the  process  of  learning  a  labyrinth,  habits  of 
movement  are  often  formed  that  are  of  no  use  whatever ;  that 
do  not  lead  to  success,  and  hence  cannot  be  guided  in  any 
sense  by  the  animal's  experience  of  their  pleasant  consequences. 
Rouse  and  Small  both  report  this  tendency  to  form  useless 
habits,  and  in  the  case  of  some  salamanders  observed  by  the 
writer,  which  never  finally  mastered  the  labyrinth  they  were 
placed  in,  habits  of  going  elaborately  wrong  would  make  their 
appearance  and  persist  for  several  days,  each  animal  re- 
maining true  to  its  individually  acquired  tendency.  The 
mere  fact  that  the  movements  were  accidentally  performed 
two  or  three  times  in  succession  created  a  persistence  in  doing 
them,  although  they  led  to  no  pleasurable  consequences  what- 
ever. 


232 


The  Animal  Mind 


§  85.    Dropping  off  Useless  Movements:    the  Puzzle-box 

Method 

The  dropping  off  of  useless  movements  is  further  illustrated 
in  those  experiments  where  animals  are  required  to  work  some 
kind  of  mechanism.  This  may  be  called  briefly  the  puzzle- 
box  method.  It  is  obviously  an  advance  in  difficulty  over  the 
labyrinth  method  in  that  it  requires  the  formation  of  a  new 


FIG.  1 6.  —  Puzzle  box  used  in  Porter's  work  on  birds.  A  B,  one  method  of  attach- 
ing string  to  latch;  C,  a  second  method.  In  the  first,  the  loop  at  B  had  to 
be  pulled;  in  the  second,  the  string  had  to  be  pushed  in. 

impulse  rather  than  the  mere  guidance  of  an  old  one;  it  does 
not  merely  direct  the  animal  in  the  performance  of  something 
that  he  would  do  anyway,  but  causes  him  to  do  something 
that  he  otherwise  would  not  do.  Yet  the  distinction  is  not 
so  fundamental  as  it  seems. 

The  puzzle-box  method  has  been  tried  with  birds,  rats,  cats, 
dogs,  raccoons,  and  monkeys.  Thorndike,  its  originator, 
made  some  experiments  of  this  type  on  chicks ;  the  animals 
were  confined  in  pens,  from  which  they  could  be  released  by 


Modification  by  Experience  233 

pecking  at  a  string  or  some  such  object.  In  other  cases,  as 
we  have  seen,  these  tests  should  be  classed  rather  with  the  laby- 
rinth method,  as  requiring  merely  that  the  chick  should  run 
out  at  a  given  definite  place  (393).  Porter  tested  English 
sparrows  with  boxes  containing  food,  which  could  be  entered 
by  pulling  a  string  fastened  to  a  latch,  or  by  pushing  the  string 
into  the  wire  netting  with  which  one  side  of  the  box  was  cov- 
ered (Fig.  1 6).  The  sparrows  learned  very  quickly;  one  of 
them  by  the  tenth  test  had  left  out  all  unnecessary  move- 
ments (344).  In  later  experiments  a  cowbird  and  a  pigeon 
also  learned  to  open  a  similar  box.  Before  beginning  the 
test  the  birds  were  accustomed  to  being  fed  in  the  box  with  the 
door  open.  Their  first  success  in  opening  the  door  lay  in 
accidentally  clawing  or  pecking  at  the  proper  point,  and  in 
later  trials  the  action  was  simplified ;  thus  the  birds  learned 
not  to  attack  other  parts  of  the  box,  to  use  the  bill  instead  of 
the  claws,  and  to  stand  on  the  floor  beside  the  box  instead  of 
hopping  upon  it.  A  point  of  some  interest  arises  in  connection 
with  the  fact  that  one  or  two  of  the  birds,  for  instance  the  male 
pigeon,  opened  the  door  in  the  simplest  possible  way,  although 
not  very  quickly,  the  first  time  they  tried  it,  and  that  these 
birds  showed  very  little  improvement  in  speed  through  sub- 
sequent trials ;  whereas  the  ones  that  had  the  most  difficulty 
about  the  first  execution  of  the  act  ultimately  reduced  their 
speed  much  below  that  of  the  others.  It  is  possible,  as  Porter 
suggests,  that  "greater  difficulty  and  therefore  more  vigorous 
activity  on  the  part  of  the  animal  in  the  initial  trials  of  any 
series  may  naturally  be  expected  to  lead  to  more  rapid  prog- 
ress in  the  later  ones"  (345).  In  Rouse's  test  of  the  pigeon 
by  the  puzzle-box  method,  it  showed  less  aptitude  than  that 
displayed  by  the  English  sparrow  (371). 

Small  tested  his  white  rats  with  two  boxes  containing  food. 
One  could  be  entered  by  digging  away  the  sawdust  which  was 


234  The  Animal  Mind 

banked  around  the  lower  end  of  the  box,  if  the  digging  was 
done  in  a  particular  place;  the  other,  by  tearing  off  strips 
of  paper  which  held  shut  a  spring  door.  The  result  of  the 
earlier  series  of  experiments  with  the  first-mentioned  box 
was  that  after  an  hour  and  a  half  on  the  first  day  one  rat 
happened  to  dig  in  the  right  place  and  entered.  The  second 
day  this  rat  took  only  eight  minutes,  and  the  thirteenth  day 
only  thirty  seconds,  to  enter.  With  the  second  box  there 
was  always  a  tendency  to  begin  by  digging,  and  even  in  the 
thirteenth  experiment,  where  the  rat  got  in  by  biting  off  the 
papers  in  fifteen  seconds,  she  began  by  two  strokes  of  dig- 
ging. In  a  later  test  with  this  box  the  rat  chanced  to  be  ex- 
tremely hungry,  and  dug  violently  for  several  seconds,  indi- 
cating a  blunting  of  the  discriminative  powers  by  hunger, 
analogous  to  that  which  we  have  found  in  very  low  animals. 
Like  Porter  subsequently,  Small  found  that  "if  a  rat  happens 
to  succeed  by  several  methods,  as,  e.g.,  biting,  clawing,  butting, 
there  is  a  strongly  marked  tendency  to  select  the  most  ex- 
peditious and  effective  method.  This  apparent  selection, 
however,  is  rather  a  matter  of  inertia  than  of  prevision." 
The  rats  were  later  trained  to  discriminate  between  the  two 
boxes,  being  sometimes  presented  with  one  and  sometimes 
with  the  other.  Such  experiments,  however,  may  properly 
be  classed  under  the  head  of  another  method  which  we  shall 
presently  discuss.  Great  individual  differences  were  found 
an^ng  the  rats :  two  of  the  four  tested  never  learned  to  get 
into  the  boxes  so  long  as  they  were  with  their  more  energetic 
companions,  but  merely  profited  by  the  activity  of  the  latter 
(386).  Watson's  puzzle-box  experiments  on  the  white  rat 
were  designed,  like  his  labyrinth  tests,  to  compare  the  powers 
of  the  adult  with  those  of  the  young.  The  results  were 
practically  the  same  as  those  of  the  labyrinth  tests,  except 
that  in  the  box  experiments,  where  mere  activity  counts  for 


Modification  by  Experience 


235 


less  than  in  a  labyrinth,  the  adult  rats  solved  the  problems 
in  less  time  than  that  occupied  by  the  young  ones  (430). 

In  Thorndike's  work  on  cats  and  dogs,  the  investigator 
placed  the  animals  themselves  in  the  boxes,  and  food  on  the 
outside,  so  that  the  problem  was  not  how  to  get  in  but  how 
to  get  out.  The  getting  out  could  be  accomplished  in  various 


FIG.  17.  — Puzzle  box  used  in  Thorndike's  experiments  on  cats. 

ways,  such  as  pulling  a  wire  loop,  clawing  a  button  around, 
pulling  a  string  at  the  top  of  the  box,  poking  a  paw  out  and 
clawing  a  string  outside,  raising  a  thumb  latch  and  pushing 
against  the  door,  and  so  on  (Fig.  17).  The  animals,  on 
being  first  put  into  the  box,  made  all  sorts  of  movements 
in  their  struggles  to  get  out;  the  right  movement  was  hit 
upon  by  accident.  Only  very  gradually,  as  the  experiment 
was  repeated  again  and  again,  were  the  useless  movements 
omitted,  until  finally  the  right  one  was  performed  at  once 
(393)-  Wesley  Mills  has  criticised  these  pioneer  experi- 
ments of  Thorndike's  on  the  ground  that  the  animals  were 
under  such  unnatural  conditions  and  in  such  an  extreme 
state  of  hunger  that  they  profited  by  experience  more  slowly 


236 


The  Animal  Mind 


than  might  otherwise  have  been  the  case  (273);  and  this 
may  have  been  to  a  certain  extent  true.  In  testing  monkeys 
with  puzzle  boxes  Thorndike  placed  the  food  on  the 
inside  and  the  monkeys  on  the  outside.  He  found  a 
marked  difference  between  the  speed  of  their  learning  and 

that  shown  by  the  cats  and 
dogs.     "  Whereas    the    latter 
were    practically    unanimous,  i 
save  in  the  cases  of  the  very  i 
easiest  performances,  in  show- 
ing a  process  of  gradual  learn- 
ing by  a  gradual  elimination 
of     unsuccessful     movements 
and   a  gradual  reinforcement 
of  the  successful  one,  these  are 

FIG.  1 8.  —  Combination  fastening  used  'unanimous,    Save    in    the    very 
in  Kinnaman's  work  on  monkeys.    ,         ,  . 

The  figures  indicate  the  order  in    hardest,  in  Showing   a   prOCCSS 
which  the  parts  of  the  combination    of     Sudden     acquisition     by    a 

rapid,  often  apparently  instan- 
taneous abandonment  of  the  unsuccessful  movements  and 
selection  of  the  appropriate  one,  which  rivals  in  suddenness 
the  selections  made  by  human  beings  in  similar  perform- 
ances "  (397).  Kinnaman  further  complicated  the  box  tests 
with  his  Macacus  monkeys  by  constructing  "combination" 
fastenings,  which  required  the  performance  of  a  set  of  actions 
in  a  certain  order,  and  found  that  these  were  mastered  by 
the  animals  (221)  (Fig.  18). 

Cole's  work  on  the  raccoon,  finally,  indicates  that  in  speed 
of  learning  this  animal  stands  "  almost  midway  between  the 
monkey  and  the  cat,"  while  "in  the  complexity  of  the  associa- 
tions it  is  able  to  form  it  stands  nearer  the  monkey."  The 
raccoons,  like  the  monkeys,  learned  combination  locks, 
although  they  did  not  learn  to  perform  the  various  move- 


Modification  by  Experience  237 

ments  involved  in  a  definite  order.  They  showed  an  inter- 
esting tendency  to  skip  at  once  to  the  movement  that  im- 
mediately preceded  the  opening  of  the  door  (82). 

The  question  arises,  as  in  the  case  of  the  labyrinth  ex- 
periments, whether,  when  the  animal  has  learned  the  proper 
movements  to  open  a  box,  he  opens  it  by  "remembering" 
the  movement ;  that  is,  by  having  some  kind  of  an  idea  or 
image  of  it  in  his  consciousness,  or  whether  we  have  to  do 
with  the  formation  of  a  habit  by  a  process  in  which  ideas 
are  at  no  time  involved.  Here,  again,  the  gradual  character 
of  the  learning  process,  where  it  is  gradual,  points  to  the 
absence  of  ideas;  a  human  being  who  had  once  hit  by 
accident  upon  the  right  way  to  open  a  lock  could  hardly  fail 
on  being  confronted  with  it  a  second  time,  at  not  too  great 
an  interval,  to  recall  an  idea  of  the  successful  movement 
and  perform  it  at  once,  without  any  unnecessary  accom- 
panying movements.  We  have  seen  an  approach  to  this 
state  of  things  in  the  monkeys;  accordingly  it  is  possible 
that  they  may  learn  by  means  of  ideas.  On  the  other  hand, 
rapid  learning,  where  the  action  is  very  simple  and  closely 
connected  with  the  animal's  instincts,  does  not  necessarily 
mean  the  presence  of  ideas;  in  certain  cases  there  may 
exist  arrangements  for  the  rapid  modification  of  an  instinctive 
mechanism  which  do  not  involve  the  production  of  images 
at  all.  The  most  we  can  say  is  that  slow  learning,  by  gradual 
elimination  of  the  useless  movements,  indicates,  so  far  as 
we  can  judge,  the  absence  of  any  guiding  idea  of  the  action. 
Other  evidence  against  the  idea  hypothesis  was  derived  by 
Thorndike  from  various  facts.  In  the  first  place  he  found 
in  the  animals  observed  by  him  an  entire  lack  of  what  has 
been  termed  inferential  imitation. 

Imitation  in  animals  has  by  some  writers,  notably  Was- 
mann,  been  classed  as  a  special  method  of  learning  by 


238  The  Animal  Mind 

experience  (426),  and  from  one  point  of  view  it  is.  But 
imitation  may  be,  as  various  authors  have  pointed  out,  of 
at  least  two  different  types.  The  first  may  be  called  in- 
stinctive imitation,  and  is  widespread  throughout  the  animal 
kingdom.  It  occurs  when  the  sight  or  sound  of  one  animal's 
performing  a  certain  act  operates  as  a  direct  stimulus,  ap- 
parently through  an  inborn  nervous  connection,  to  the  per- 
formance of  a  similar  act  by  another  animal.  "If,"  says 
Lloyd  Morgan,  "one  of  a  group  of  chicks  learns  by  casual 
experience  to  drink  from  a  tin  of  water,  others  will  run  up 
and  peck  at  the  water  and  will  themselves  drink.  A  hen 
teaches  her  little  ones  to  pick  up  grain  or  other  food  by  peck- 
ing on  the  ground  and  dropping  suitable  materials  before 
them,  the  chicks  seeming  to  imitate  her  actions.  .  .  .  In- 
stinctive actions,  such  as  scratching  the  ground,  are  performed 
earlier  if  imitation  be  not  excluded  "  (281,  pp.  166-167). 
Imitation  in  this  sense  is  hardly  so  much  a  method  of  learn- 
ing by  experience  as  a  method  of  supplying  experience. 
An  animal  may  perform  an  act  the  first  time  because,  through 
inherited  nervous  connections,  the  sight  of  another  animal's 
performing  it  acts  as  a  stimulus.  But  it  will  continue  to 
perform  the  act,  in  the  absence  of  any  copy  to  imitate,  only 
if  the  act  is  itself  an  instinctive  one,  like  drinking  in  birds, 
or  becomes  permanent  by  reason  of  its  consequences,  just  as 
would  be  the  case  if  its  first  performance  had  been  accidental 
rather  than  imitative.  As  a  matter  of  fact,  instinctive  imita- 
tion seems  usually  to  be  concerned  with  actions  themselves 
instinctive. 

Inferential  imitation,  or  what  Morgan  calls  reflective 
imitation,  is  a  different  affair.  It  is  the  case  where  an  animal, 
watching  another  one  go  through  an  action  and  observing 
the  consequences,  is  led  to  perform  a  similar  act  from  a 
desire  to  bring  about  the  same  result.  The  most  natural 


Modification  by  Experience  239 

description  of  the  subjective  side  of  this  process  in  a  human 
being  would  be  to  say  that  the  sight  of  the  other  individual's 
behavior  "suggests  the  idea"  of  similar  behavior  on  one's 
own  part.  Inferential  imitation  would  then  not  differ 
fundamentally  from  any  other  case  of  learning  by  ideas. 
Now  Thorndike,  in  his  experiments  on  cats  and  dogs,  found, 
as  we  have  said,  no  evidence  of  this  type  of  imitation.  A 
cat  put  in  a  puzzle  box  did  not  learn  the  way  out  any  sooner 
for  watching,  even  repeatedly,  the  performances  of  a  cat 
that  knew  how  to  get  out.  With  monkeys,  Thorndike's 
most  extensive  tests  were  made  to  find  whether  the  animal 
would  learn  to  open  a  box  from  seeing  the  experimenter 
himself  do  it,  and  his  results  were  again,  on  the  whole, 
negative  (393).  Small's  white  rats  also  showed  no  ability 
to  profit  by  each  other's  experience  in  this  way.  One  of 
each  of  the  pairs  first  experimented  on  solved  the  problems 
presented;  the  other,  instead  of  either  attacking  them  for 
itself  or  learning  by  watching  the  successful  one,  contented 
itself  with  stealing  the  food  secured  by  the  latter  (386). 
Imitation,  according  to  Yerkes,  plays  no  considerable  role 
in  the  learning  processes  of  the  dancing  mouse  (469). 

On  the  other  hand,  Kinnaman's  monkeys  did  give  some 
indications  of  learning  by  inferential  imitation.  In  one 
case,  the  box  had  to  be  opened  by  pulling  out  a  plug.  One 
monkey  failed  to  work  the  mechanism,  and  gave  up  in  despair. 
Another  one  then  came  out  of  the  cage,  the  first  one  follow- 
ing. Number  two  went  to  the  box,  seized  the  end  of  the  plug 
with  its  teeth  and  pulled  it  out.  The  box  was  set  again, 
and  monkey  number  one  rushed  to  it,  seized  the  plug  as 
number  two  had  done,  and  got  the  food.  She  immediately 
repeated  the  act  eight  times.  A  second  and  similar  observa- 
tion was  made  where  the  mechanism  was  a  lever  (221). 
Hobhouse  found  that  cats,  dogs,  elephants,  and  monkeys 


240  The  Animal  Mind 

were  aided  in  their  learning  processes  if  he  "showed"  them 
how  to  do  the  thing  (177).  Whether  this  was  inferential 
imitation  in  the  sense  that  they  got  the  idea  of  the  action 
and  of  its  result  by  watching  him,  or  whether  they  were 
merely  aided  in  focussing  their  attention  on  the  important 
object,  the  string,  hook,  or  lever,  it  is  difficult  to  be  sure. 

Berry  found  that  the  white  rats  he  experimented  on 
manifested  a  type  of  imitative  behavior  which  he  is  inclined 
to  regard  as  intermediate  between  instinctive  and  fully  in- 
ferential imitation.  "When  two  rats  were  put  into  the  box 
together,"  he  says,  "one  rat  being  trained  to  get  out  of  the 
box  and  the  other  untrained,  at  first  they  were  indifferent 
to  each  other's  presence,  but  as  the  untrained  rat  observed 
that  the  other  one  was  able  to  get  out  while  he  was  not,  a 
gradual  change  took  place.  The  untrained  rat  began  to 
watch  the  other's  movements  closely;  he  followed  him  all 
about  the  cage,  standing  up  on  his  hind  legs  beside  him  at 
the  string  and  pulling  it  after  he  had  pulled  it,  etc.  We 
also  saw  that  when  he  was  put  back,  the  immediate  vicinity 
of  the  loop  was  the  point  of  greatest  interest  for  him,  and 
that  he  tried  to  get  out  by  working  at  the  spot  where  he  had 
seen  the  trained  rat  try  "  (26). 

Now,  so  far  as  the  light  cast  by  this  evidence  for  and  against 
inferential  imitation  or  the  presence  of  ideas  in  the  animal 

ind  is  concerned,  the  matter  seems  to  stand  as  follows. 

e  cannot  be  sure  that  Kinnaman's  monkeys  really  had 
an  idea  of  the  proper  action  suggested  to  them  by  seeing 
their  companions  perform  it ;  the  case  might  have  been  one 
of  instinctive  imitation,  taking  here  a  form  more  elaborate 
than  was  seen  in  cats  and  dogs  because  more  complicated 
movements  are  natural  to  the  monkey  than  to  the  lower 
mammals.  If  it  is  certain  that  Berry's  uneducated  rat 
began  to  watch  the  actions  of  the  educated  one  more  closely 


Modification  by  Experience  241 

as  a  result  of  its  observation  that  the  latter  succeeded  where 
it  failed,  then,  although  the  imitation  was  confined  merely 
to  investigating  the  same  general  locality  as  that  attacked 
by  the  trained  rat,  and  not  extended  to  an  actual  performalSce 
of  the  movement,  it  would  seem  to  be  inferential  in  type. 
But  precisely  this  certainty  is  apparently  rather  hard  to 
attain.  Again,  when  an  animal  is  assisted  in  learning  by 
watching  a  human  being  who  undertakes  to  show  him  how, 
is  he  given  an  idea  of  the  act  and  its  results,  or  does  he  merely 
have  his  attention  called  to  the  important  part  of  the  mechan- 
ism to  be  worked?  True  inferential  imitation  is  hard  to 
prove.  On  the  other  hand,  the  failure  of  an  animal  to  show 
inferential  imitation  does  not  mean  that  the  animal  cannot 
have  ideas,  f  We  cannot  conclude  that  an  animal  is  incapable 
of  ideas  because  it  does  not  have  them  suggested  to  it  under 
circumstances  that  would  suggest  them  to  our  minds.) 

Again,  Thorndike  noticed  that  while  after  a  time  the  cats 
that  had  been  caused  to  go  into  the  box  and  let  themselves 
out  before  being  fed  would  go  into  the  box  of  their  own 
accord,  cats  that  had  been  from  the  first  dropped  into  the 
box  at  the  top  did  not  learn  to  go  into  it  of  themselves.  He 
argues  that  if  the  cat  had  been  able  to  have  the  idea  of  being 
in  the  box,  as  a  necessary  prelude  to  food,  it  would  have 
been  able  to  pass  from  the  idea  of  being  dropped  in  to  that 
of  going  in  itself.  Further,  he  found  that  he  could  not  train 
the  animals  to  do  a  certain  act  by  forcibly  putting  them 
through  it,  and  concludes  that  if  they  had  been  capable  of 
having  the  idea  of  the  act,  it  would  have  been  suggested  to 
them  by  this  process  (393).  But  in  each  of  these  cases, 
the  experience  which  is  supposed  to  give  the  animal  the  idea 
of  the  act  is  one  that  has  some  points  of  likeness  with  the 
act  and  other  points  of  decided  difference.  The  experience 
of  seeing  another  animal  perform  a  movement  is  very  different 


242  The  Animal  Mind 

from  that  of  performing  it  oneself ;  the  experience  of  being 
picked  up  and  dropped  into  a  box  is  very  different  from  that 
of  walking  in  through  the  door,  and  the  experience  of  being 
forcibly  held  and  put  through  a  movement  differs  from  do- 
ing it  without  restraint.  To  the  human  mind,  accustomed  to 
more  analysis  of  its  own  experiences,  one  of  these  would 
suggest  the  other,  but  we  cannot  argue  that  because  such  an 
association  is  not  made  in  the  animal's  mind,  therefore  the 
latter  is  incapable  of  ideas,  any  more  than  we  could  conclude 
a  total  absence  of  ideas  in  the  consciousness  of  a  man  to 
whom  a  primrose  by  the  river's  brim  does  not  suggest  thoughts 
of  the  moral  government  of  the  Universe. 

Moreover,  the  raccoon,  according  to  Cole,  presents  some 
of  these  very  indications  of  ideas  in  its  learning  processes. 
In  the  first  place,  the  raccoons,  unlike  Thorndike's  cats, 
did  "run  back  into  boxes  into  which  they  had  hitherto  been 
lifted."  They  were  picked  up  by  the  nape  of  the  neck 
and  dropped  into  the  boxes.  On  the  thirty-third  trial  in 
the  case  of  one  raccoon,  "she  turned  .  .  .  and  went  quickly 
back  into  the  box.  She  opened  the  door  in  six  seconds, 
came  out,  was  fed  for  a  moment  from  the  bottle,  and  then 
immediately  re-entered  the  box."  It  is,  then,  at  least  possible 
that  the  idea  of  being  in  the  box  was  suggested  to  them  from 
the  experience  of  being  dropped  in.  In  the  second  place, 
unlike  Thorndike's  subjects,  the  raccoon  learned  to  work  a 
fastening  by  being  put  through  it.  For  example,  raccoon 
number  two  had  failed  to  learn  to  raise  a  horizontal  hook. 
"To  make  it  a  certain  failure,  I  waited  thirty-two  minutes 
while  he  worked  steadily.  I  put  him  through  five  times 
by  raising  the  hook  with  his  nose.  He  then  succeeded  in 
three  and  four-tenths  seconds,  then  in  seven  and  two-tenths, 
and  so  on."  Other  examples  are  not  quite  so  clear-cut  as 
this,  but  there  is  ample  evidence  that  putting  the  raccoon 


Modification  by  Experience  243 

through  the  proper  movement  greatly  facilitated  the  learning 
process.  Further  evidence  of  the  presence  of  ideas  in  the 
raccoon's  mind  will  be  considered  later  (82).  Yerkes  has 
found  that  the  dancing  mouse  is  aided  in  learning  by  being 
put  through  the  act  (469).  Hobhouse  thinks  that  his  cats, 
dogs,  elephants,  and  monkeys  showed  that  their  actions  were 
guided  by  an  idea  of  the  result,  instead  of  being  merely  ac- 
quired reactions  to  stimuli,  because  they  varied  the  means 
to  the  end.  "In  opening  the  sideboard  drawer,  Jack 
[a  dog]  not  merely  pulls,  but  learns  for  himself  how  to  get 
his  head  into  the  drawer  without  shutting  it  again,  altering 
the  method  when  he  once  hurts  himself,  and  finding  another. 
So  again,  I  have  seen  him,  when  standing  up  to  pull  open  the 
door  of  his  box  by  means  of  a  wire,  accidentally  pushing  it 
with  his  paws  again  as  he  let  go.  At  a  second  trial  he  was 
careful  to  avoid  this,  dropping  the  wire  and  pushing  his 
nose  in  as  soon  as  there  was  room.  Similarly,  I  have  seen 
the  elephant  shift  the  box  that  she  was  opening  when  she 
had  found  that  in  a  certain  position  the  door  would  slam  to 
again  before  she  could  get  her  trunk  in."  These  bits  of 
behavior,  in  Hobhouse's  opinion,  indicate  that  the  animals 
have  ideas  of  the  changes  they  wish  to  bring  about  (177). 
While  it  may  be  rash  to  assert  of  any  particular  higher 
animal  that  its  consciousness  never  'contains  ideas,  yet  the 
slow  acquisition  observed  in  many  of  these  experimental 
tests  certainly  gives  evidence  of  a  process  of  learning  whose 
essence  consists  simply  in  the  gradual  dropping  off  of  un- 
necessary movements.  Upon  the  nature  of  this  process, 
psychology  can  throw  little  light.  Thorndike  declares  that 
the  successful  movement  is  "stamped  in"  because  pleasure 
results  from  it,  while  the  unsuccessful  movements  are 
"stamped  out"  because  no  pleasure  results.  The  terms 
"  stamping  in  "  and  "  out "  must  refer  to  some  effect  upon 


244  The  Animal  Mind 

the  nervous  system,  and  of  this  effect  we  know  no  more  than 
that  it  exists.  Jennings  says  that  the  disturbance  set  up  in 
the  organism  by  the  stimulus,  by  hunger  or  confinement, 
as  the  case  may  be,  not  finding  an  outlet  by  one  path  of 
discharge,  seeks  others  in  succession  until  one  is  found  which 
relieves  the  disturbed  condition.  This,  we  have  seen,  he 
and  others  have  found  to  be  the  case  in  very  low  forms  of 
animal  life.  But  the  crucial  part  of  the  phenomena  we  are 
now  considering  is  described  in  the  following  sentence : 
"  After  repetition  of  this  course  of  events,  the  change  which 
leads  to  relief  is  reached  more  directly,  as  a  result  of  the 
law  of  the  readier  resolution  of  physiological  states  after 
repetition  "  (208).  And  that  is  all  we  know  of  the  matter. 
But  we  may  well  note  the  probability  that  a  habit,  in  the 
sense  of  a  fixed  way  of  action  not  innate  in  the  individual, 
may  originate  in  two  ways :  first,  by  the  loss  of  conscious 
control  in  the  case  of  a  set  of  actions  originally  voluntary 
and  guided  by  ideas;  and  second,  by  the  gradual  increase 
of  speed  and  accuracy  in  the  performance  of  a  series  of 
actions,  never  at  any  time  guided  by  anything  but  external 
stimuli.  In  our  own  experience,  the  first  kind  of  habit 
formation  has  been  of  so  much  interest  that  it  has  diverted 
attention  from  the  second.  Yet  the  latter  is  shown  con- 
stantly in  the  growth  of  skill  that  comes  through  the  mere 
repetition  of  a  series  of  movements,  apart  from  the  "  knowing 
how,"  which  means  conscious  control. 

§  86.   The  Psychic  Aspect  of  dropping  off  Useless  Move- 
ments 

The  conscious  aspect  of  learning  by  dropping  off  useless 
movements  must  consist  largely  in  the  mere  shortening  of 
a  period  of  unpleasantness  and  unrest.  The  useless  move- 
ments are  unpleasant,  the  successful  one  brings  pleasure; 


Modification  by  Experience  245 

when  the  latter  comes  to  be  performed  at  once,  the  con- 
sciousness accompanying  must  be  wholly  pleasant.  Further,  k 
the  puzzle-box  experiments  differ  from  the  labyrinth  ex-  \ 
periments  in  that  the  successful  movement  is  not  merely 
turning  in  one  direction  rather  than  another,  a  habit  which 
might  ultimately  become  perfectly  independent  of  external 
stimuli;  but  a  reaction  upon  a  particular  object.  It  is 
evident  that  in  the  course  of  learning,  this  object,  at  the 
outset  unnoticed,  must  come  to  stand  in  the  centre  of  the 
animal's  consciousness,  the  focus  of  its  attention,  as  soon 
as  it  is  perceived  at  all.  Such  a  change  would  constitute 
another  feature  of  the  consciousness  accompanying  this 
form  of  learning,  in  addition  to  the  diminishing  unpleasant- 
ness which  goes  along  with  the  dropping  off  of  unnecessary 
movements.  A  special  aspect  of  experiments  on  imitation 
is  connected  with  this  process  of  learning  to  focus  attention 
on  ike  proper  object.  Can  an  animal  have  his  attention 
" called"  to  the  object  by  watching  some  one  else  operating 
on  it,  or  must  the  object  gain  its  power  to  determine  reaction 
solely  by  the  animal's  own  experience  of  the  consequences 
connected  with  it?  Hobhouse's  experiments  on  dogs,  cats, 
and  other  animal  subjects  led  him  to  the  conclusion  that  \ 
watching  his  performance  did  materially  assist  the  animal  \ 
in  this  respect.  His  method  was  to  perform  the  action  of 
pulling  a  lever  or  string  repeatedly  while  the  animal  was 
watching;  and  that  this  process  facilitated  learning  he 
thinks  evident  from  the  fact  that  after  it  had  been  several 
times  repeated,  the  behavior  of  the  animal  changed  in  a 
marked  degree;  instead  of  being  random,  it  was  definitely 
directed  at  the  proper  object.  The  suddenness  of  the  transi- 
tion from  random  to  definite  behavior  indicates,  in  Hobhouse's 
opinion,  the  effect  of  being  shown.  Thus,  for  example, 
his  dog  Jack  "after  being  once  shown,  .  .  .  learnt  to  pull 


246  The  Animal  Mind 

a  stopper  out  of  a  jar  with  his  teeth.  The  stopper  fitted 
into  a  large  round  glass  jar,  and  could  be  lifted  with  the 
teeth  by  a  projecting  peg.  I  lifted  it  out  for  him  once,  and 
left  him  to  deal  with  the  jar,  which  he  did  by  knocking  it 
over  and  rolling  it  all  about  the  room  until  the  meat  was 
jerked  out.  At  the  second  trial  he  pulled  at  the  stopper  him- 
self with  his  teeth;  and  he  repeated  this  many  times"  (177). 
A  cat  with  which  the  writer  is  acquainted  stands  on  his  hind 
legs  and  touches  a  door  handle  with  his  paw  when  he  wishes 
to  be  let  out.  He  has  never  succeeded  in  letting  himself 
out  by  any  such  method.  It  is  possible  that  the  habit  may 
have  been  acquired  from  the  fact  that  the  door  is  sometimes 
opened  for  him  after  he  has  done  so ;  but  this  is  by  no  means 
always  the  case.  He  is  often  left  to  mew  for  some  time 
after  he  has  pawed  the  handle.  There  is,  then,  the  pos- 
sibility that  observing  human  beings  open  doors  may  have 
caused  the  handle  to  occupy  the  focus  of  his  attention; 
but  one's  attitude  toward  this  hypothesis  should  be  ex- 
tremely cautious. 


CHAPTER  XI 

THE  MODIFICATION  OF  CONSCIOUS  PROCESSES  BY  INDI- 
VIDUAL EXPERIENCE  (continued) 

§  87.   The  Inhibition  of  Instinct 

IN  still  another  form  of  experiment  that  has  been  devised 
to  study  the  ways  in  which  animals  learn  by  experience,  the 
object  has  been  to  secure  the  complete  inhibition  of  an  in- 
stinctive action.  Obviously  two  factors  will  come  into  play 
here, — the  strength  of  the  instinct  and  the  force  of  the  modi- 
fying experience.  The  latter  factor  we  might  suppose  to  be 
strongest  when  the  performance  of  an  instinctive  action 
could  be  made  attended  with  pain,  and  less  strong  when 
the  performance  of  some  action  opposed  to  instinct  has  been 
found  to  be  accompanied  by  pleasure. 

The. first  case  we  find  apparently  illustrated  by  Morgan's 
chick  in  his  dealings  with  a  bee ;  he  needed  but  one  experi- 
ence with  that  insect  to  inhibit  entirely,  the  next  day,  his 
instinct  to  peck  at  it  (281,  p.  53).  On  the  other  hand, 
Bethe  denied  consciousness  to  the  crab  because,  although 
every  time  it  went  into  the  darkest  corner  of  the  aquarium 
it  was  seized  by  a  cephalopod  lurking  there,  it  did  not  in  six 
experiences  learn  to  inhibit  its  negative  phototropism ;  nor 
did  the  crabs  learn  not  to  snap  at  meat,  though  several  times 
when  they  did  so  they  were  seized  by  the  experimenter  (28). 
The  case  of  the  crabs  is  not,  however,  fairly  comparable 
with  that  of  the  chick,  for  the  latter  was  not  really  obliged 
to  inhibit  his  pecking  instinct  altogether,  but  only  to  direct 

247 


248  The  Animal  Mind 

it  away  from  a  certain  object,  while  the  crabs  had  no  outlet 
at  all  for  their  photic  and  nutritive  instincts.  Very  likely 
a  longer  course  of  training  than  that  employed  by  Bethe 
might  have  succeeded  in  suppressing  the  instincts.  The 
chick's  case  really  belongs  to  a  form  of  learning  which  we 
shall  consider  later  on;  that  where  the  inhibition  depends 
on  the  discrimination  of  different  stimuli.  The  purest 
instance  of  the  kind  of  modification  of  behavior  by  ex- 
perience at  present  concerning  us  is  furnished  by  Mobius's 
experiments  with  the  pike  (276),  afterward  repeated  by 
Triplett  with  perch  (407).  The  pike  was  kept  in  one  half  of 
an  aquarium,  separated  by  a  glass  screen  from  the  other 
half,  in  which  minnows  were  swimming  about.  The  pike 
naturally  dashed  at  them,  and  received  a  bump  on  its  nose 
whenever  it  did  so.  After  a  considerable  period  of  this 
sort  of  experience,  the  glass  screen  was  removed,  and  the 
minnows  were  allowed  to  swim  freely  around  the  pike, 
when  it  was  found  that  the  latter's  instinct  to  seize  them 
had  been  Wholly  inhibited  by  the  disagreeable  consequences  of 
such  action.  Triplett's  description  of  the  occasional  struggles 
of  the  instinct  to  assert  itself  is  extremely  interesting.  An 
analogous  case  is  offered  by  Goldsmith's  account  of  the 
shell-inhabiting  fish,  Gobius,  which,  when  a  glass  partition 
was  placed  between  it  and  its  shell  domicile,  dashed  against 
the  glass  for  a  time,  but  after  three  and  a  half  hours  went 
around  it,  and  the  next  day  did  so  after  only  a  quarter  of  an 
hour's  unsuccessful  attempting  to  get  through  the  parti- 
tion (146). 

The  second  kind  of  training  in  the  inhibition  of  an  in- 
stinct, where  the  performance  of  an  action  opposed  to  instinct 
is  made  to  produce  pleasure,  is  illustrated  by  Spaulding's 
work  on  "Association  in  Hermit  Crabs."  He  found  that  these 
animals,  which  are  positively  phototropic,  could  be  trained 


Modification  by  Experience  249 

to  go  into  the  darkened  part  of  the  aquarium  to  get  food, 
and  finally  to  do  so  even  if  no  food  was  there  (389).  Es- 
pecially striking  as  an  example  of  this  kind  of  learning  is 
the  behavior  of  the  insect  called  the  water  scorpion  in  the 
experiments  of  Holmes  mentioned  on  page  179.  With  its 
head  directly  away  from  the  light,  and  the  right  eye  blackened, 
the  natural  tendency  of  this  positively  phototropic  insect  was 
to  turn  to  the  left.  Yet  after  a  sufficient  amount  of  training 
in  a  position  where  the  natural  tendency  was  to  turn  toward 
the  right,  the  animal,  on  being  replaced  with  its  back  to  the 
light,  turned  toward  the  right,  an  action  directly  contrary 
to  instinct  having  been  thus  brought  about  by  experience, 
as  Holmes  thinks,  and  as  we  may  certainly  conjecture,  of  its 
pleasurable  consequences  (186).  When  the  " flight-reflex" 
comes  gradually  to  be  inhibited  in  animals  that  are  being 
tamed,  we  have  another  instance,  of  this  type  of  learning 
(e.g.,  106). 

The  chief  psychological  question  involved  in  the  con- 
sideration of  that  form  of  learning  by  experience  which 
involves  the  inhibition  or  reversal  of  an  instinct  is  whether 
there  is  in  the  animal's  mind  an  actual  representation  of 
the  effects  of  the  actions  which  constitute  the  animal's  train- 
ing. Does  the  pike,  confronted  with  the  minnow,  recall 
the  bump  on  its  nose?  Where  the  learning  is  very  rapid, 
this  always  remains  possible.  Where  the  process  is  slower, 
however,  the  simpler  hypothesis  would  be  that  the  pleasure 
and  pain  of  the  results  operate  directly  on  the  animal's 
tendencies  to  move,  without  the  intervention  of  images. 
In  the  experiments  where  the  results  are  painful,  the  stimulus 
at  first  produces,  through  the  animal's  inherited  nervous 
connections,  a  movement  toward  it.  This  movement,  un- 
der the  peculiar  circumstances  of  the  case,  occasions  pain, 
and  pain  brings  about  a  negative  reaction  of  withdrawal. 


250  The  Animal  Mind 

It  is  possible  that,  as  a  consequence  of  the  general  tendency 
of  the  nervous  system  to  establish  short-cuts,  after  repetition 
of  this  experience,  the  appearance  of  the  stimulus  may  stir 
up  the  negative  reaction  soon  enough  to  inhibit  the  positive 
one  altogether;  through  the  operation  of  the  same  law 
whereby  in  learning  a  foreign  language  we  first  pass  to  the 
meaning  of  a  word  by  way  of  the  sound  of  the  English  word, 
but  later  make  the  association  without  any  intervening  link. 
On  the  psychic  side,  the  object  which  was  at  first  agreeable 
has  simply  become  disagreeable.  As  for  the  cases  where  the 
instinct  has  been  reversed  by  means  of  pleasurable  conse- 
quences, the  training  of  the  hermit  crab  was  accomplished  by 
pitting  a  stronger  against  a  weaker  instinct.  Nothing  but  the 
natural  victory  of  the  stronger  innate  tendency  to  move  was 
required  to  make  the  crab  go  into  the  dark  part  of  the  aqua- 
rium to  be  fed,  when  the  food  was  actually  there ;  but  what 
made  it  continue  to  do  so  when  the  food  was  removed? 
The  representation  of  the  resulting  pleasure,  Spaulding  says ; 
shall  we  admit  this,  or  confine  ourselves  to  physiological 
terms,  and  say  that  the  nervous  energy  involved  in  the  sight 
of  the  dark  corner  has  come  to  find  its  natural  outlet  in 
movements  toward  that  corner,  through  the  repetition  of 
these  movements  as  a  result  of  the  operation  of  the  stronger 
food  instinct?  The  case  of  the  water  scorpion  certainly 
suggests  rather  the  operation  of  a  blind  habit  than  the  effect 
of  any  representation  of  pleasure.  Here  the  light-seeking 
instinct  is,  as  it  were,  pitted  against  itself ;  shall  we  say  that 
the  animal,  guided  by  a  representation  of  the  pleasure  it 
has  previously  derived  from  turning  to  the  left,  does  so  now, 
when  the  slightest  turn  to  the  right  would  actually  give  it 
that  pleasure?  Possibly,  but  the  behavior  looks  more  like 
the  working  of  a  mechanical  habit  of  turning. 


Modification  by  Experience  251 

§  88.  Inhibition  involving  Discrimination  of  Successive 
Stimuli 

In  other  experiments  requiring  the  inhibition  of  an 
instinct,  the  animal  is  caused  to  discriminate  between  two 
nearly  similar  stimuli,  to  execute  the  action  when  one  is 
presented  and  to  inhibit  it  when  the  other  one  appears. 
Such  tests,  as  we  have  seen,  are  very  commonly  adopted 
to  investigate  sensory  discrimination.  The  principle  is  the 
same  whether  the  action  to  be  inhibited  is  an  instinct  or  an 
acquired  habit.  The  experiments  may  be  divided  into  two 
classes:  in  the  first  only  one  stimulus  is  given  at  a  time, 
and  the  animal  in  consequence  is  sometimes  required  to 
inhibit  its  response  entirely;  in  the  second,  two  or  more 
stimuli  are  given  simultaneously ,  and  the  animal  simply 
has  to  choose  among  them  on  the  basis  of  its  past  experiences. 
Experiments  belonging  in  the  former  class  were  successfully 
performed  by  Thorndike  with  Cebus  monkeys.  Both  of  his 
subjects  learned  to  come  down  to  the  bottom  of  the  cage 
to  be  fed  when  the  experimenter  took  a  piece  of  food  in  his 
left  hand,  and  to  stay  up  when  he  took  it  with  his  right  hand, 
by  being  fed  in  the  first  case  and  not  in  the  second.  One 
of  the  monkeys  learned  to  discriminate  in  like  manner  be- 
tween cards  carrying  different  figures;  the  other  one  failed 
with  the  cards  and  learned  to  react  or  inhibit  reaction  only 
in  connection  with  different  movements  of  Thorndike's 
hands  (397).  Professor  Bentley  and  the  writer  made  some 
experiments  on  the  chub  by  this  method,  which  gave  wholly 
negative  results.  The  red  and  green  forceps,  each  con- 
taining food,  were  plunged  one  at  a  time  into  the  water; 
the  fish  was  allowed  to  get  the  food  from  the  red  forceps, 
but  the  green  ones  were  withdrawn  before  it  had  a  chance 
to  bite.  The  time  which  the  fish  took  to  rise  and  snap  at 


252  The  Animal  Mind 

the  forceps  was  measured  by  a  stop-watch,  and  in  the  course 
of  131  experiments  the  animal  was  not  found  to  rise  to  the 
green  any  less  promptly  than  to  the  red.  In  other  words, 
no  tendency  to  inhibit  reaction  to  the  green  was  shown, 
although  our  later  experiments  proved  that  the  fish 'could 
distinguish  the  two  (421).  Apart  from  the  difference  in 
.intellectual  level  between  the  fish  and  the  monkey,  it  is 
probable  that  the  food- taking  instinct  was  stronger  in  the 
former,  which  came  directly  from  the  wild  state,  where  it 
could  afford  to  lose  no  chances  of  nourishment.  Dahl's 
observation  that  the  spider  Attus  arcuatus  refused  to  take 
house  flies  after  having  been  presented  with  one  smeared 
with  oil  of  turpentine,  although  it  seized  a  gnat,  is  also  a  case 
of  inhibition  involving  discrimination  of  successively  offered 
stimuli  (88).  Cole,  in  his  very  interesting  experiments  on 
the  raccoon,  raises  the  question  whether  discriminations  of 
this  type  do  not  involve  memory  images,  and  answers  it  in 
the  affirmative.  He  used  the  method  to  test  discrimination 
of  colors,  tones,  forms,  and  sizes;  the  results  have  been 
noted  in  earlier  chapters.  The  cards  used  were  placed  on 
levers  so  that  by  a  touch  they  could  be  pushed  up  and 
down.  The  animals  learned  to  climb  up  for  food 
when  one  of  two  differently  colored  cards  was  shown,  and 
to  stay  down  when  the  other  one  appeared;  to  distinguish 
in  a  similar  way  between  a  high  and  a  low  tone,  between 
a  round  and  a  square  card,  and  between  a  card  6|  x  6J 
inches  and  one  4^  x  4^  inches  square.  Of  course  the  action 
of  climbing  up  was  not  itself  purely  instinctive,  but  had 
become  associated  with  the  food  instinct.  The  raccoons 
also  hit  upon  the  trick  of  clawing  up  the  cards  themselves, 
and  if  the  one  that  appeared  was  the  " no-food"  card,  they 
would  either  claw  it  down  again  and  pull  up  the  other,  or 
proceed  at  once  to  pull  up  the  other,  leaving  the  "no-food" 


Modification  by  Experience  253 

one  also  up.  Since  the  cards  were  shown  successively,  Cole 
concludes  that  "remembrance  of  the  card  just  shown  was 
required  for  a  successful  response."  "Why,"  he  asks, 
"should  the  animal  put  the  red  card  down  if  it  did  not  fail 
to  correspond  with  some  image  he  had  in  mind,  and  why 
when  he  put  the  green  up  should  he  leave  it  up  and  go  up  on 
the  high  box  for  food  if  the  green  did  not  correspond  with 
some  image  he  had  in  mind?"  (82).  It  seems  to  the  writer 
that  the  supposition  of  an  image  is  unnecessary,  except 
possibly  in  the  experiments  requiring  discrimination  of 
sizes.  It  is  perfectly  possible,  as  we  know  from  our  own 
experience,  to  react  to  one  stimulus  and  not  to  another  with- 
out going  through  a  comparison  of  the  two,  unless  the  differ- 
ence between  them  is  merely  one  of  degree.  It  might  have 
been  possible  for  a  human  being  to  discriminate  between 
the  larger  and  the  smaller  cards  only  by  calling  up  a  memory 
image  of  the  card  not  shown  and  comparing  it  with  the  one 
before  him ;  it  surely  would  not  have  been  necessary  for  him 
\  to  use  images  in  the  reactions  to  colors,  forms,  and  tones. 
I  And  if  a  human  being,  accustomed  to  much  dependence  on 
\  memory  ideas,  could  get  on  without  them  here,  surely  a 
\  raccoon  could.  Even  in  judgments  of  degree,  all  laboratory 
\psychologists  know  that  human  beings  have  a  strong  tendency 
\to  make  absolute  rather  than  comparative  judgments,  and 
use  memory  ideas  but  little.  Better  evidence  of  the  use  of 
images  is  furnished  by  the  following  method:  "Three 
levers  were  placed  on  the  displayer.  'One,  on  being  raised, 
displayed  white,  another  orange,  another  blue.  The  plan 
was  to  display  white,  orange,  and  blue  consecutively,  then 
to  display  the  same  blue  three  times.  I  fed  the  animal  if 
he  climbed  upon  the  high  box  on  being  shown  the  series 
white,  orange,  blue,  and  did  not  feed  him  after  the  series 
blue,  blue,  blue."  That  is,  the  stimulus  immediately  pre- 


254  The  Animal  Mind 

ceding  the  reaction  was  the  same  in  both  cases.  The  differ- 
ence lay  in  the  foregoing  stimuli.  The  series  "  white,  blue, 
red,  food,  and  red,  red,  red,  no  food"  was  also  used.  The 
raccoons  learned  to  respond  properly,  ''though,"  Cole  con- 
tinues, "I  never  completely  inhibited  the  animals'  tendency 
to  start  up  on  seeing  white  or  blue,  which  were  precursors  of 
the  red  which  meant  food.  Thus  the  animals  all  anticipated 
red  on  seeing  its  precursors,  which  in  itself  seems  good 
evidence  of  ideation.  Many  times,  however,  they  turned 
back  after  starting  at  blue  or  white  and  looked  for  the  red, 
then  climbed  up  once  more,  thus  showing  that  the  red  was 
not  a  neglected  element  of  the  situation,  but  an  expected 
color  which  they  generally  waited  to  see,  but  sometimes 
were  too  eager  to  wait  for."  Two  of  the  raccoons  had  been 
previously  trained  in  two-color  series,  while  one  had  ex- 
perienced only  the  three-color  series.  The  former  showed 
a  decided  tendency  to  go  up  at  the  second  color  when  there 
were  three.  The  latter  had  been  trained  first  on  the  series 
"white,  orange,  blue,  food;  blue,  blue,  blue,  no  food;" 
then  on  the  series  "white,  blue,  red,  food;  red,  red,  red,  no 
food."  "Although  blue,  his  former  food  signal,  was,"  in 
the  second  series,  "placed  second  as  a  no-food  color,  he  made 
the  mistake  of  reacting  to  it  only  ten  times  in  the  first  fifty, 
because  it  was  not  third,  while  he  did  go  up  to  the  final  'no- 
food  '  red  twenty-seven  times  because  it  was  third.  It  seems 
certain,  therefore,  that  raccoons  are  able  to  learn  to  dis- 
tinguish one  object  or  movement  from  two  and  two  from 
three,  a  species  of  counting  not  different  from  that  which 
anthropologists  ascribe  to  primitive  man."  Certain  details 
of  the  behavior  of  the  raccoons  in  these  tests  are  significant. 
"Each  one,  on  seeing  the  first  red,  would  drop  down  from 
a  position  with  both  front  paws  on  the  front  board  to  stand 
on  all  fours  in  front  of  it  and  merely  glance  up  at  the  sue- 


Modification  by  Experience  255 

ceeding  reds.  As  soon  as  the  white  appeared,  however, 
the  animal  would  lean  up  against  the  front  board,  claw 
down  the  white  and  blue,  but  never  the  final  red." 

Now  Cole  thinks  that  the  learning  of  this  trick  by  the  rac- 
coons proved  that  "the  animal  retains  an  image  of  the  cards 
which  just  preceded  red."  The  only  alternate  supposition 
seems  to  him  to  be  that  they  always  reacted  to  the  number 
of  the  card  in  the  series,  which,  if  the  series  were  irregularly 
given,  would  not  have  been  the  same  in  successive  trials.  To 
suggest  one's  own  interpretation  of  animal  behavior  that  one 
has  not  seen,  in  the  place  of  the  experimenter's  interpretation, 
requires  some  temerity,  but  to  the  present  writer  the  most 
natural  way  of  accounting  for  the  raccoon's  performances 
would  be  the  supposition  that  in  the  series  white,  blue,  red, 
for  instance,  at  the  end  of  which  they  were  fed,  the  occurrence 
of  white  threw  them  into  a  state  of  expectancy,  of  readiness 
to  climb  up  on  the  box ;  this  was  heightened  by  the  blue,  and 
finally  "discharged"  into  action  by  the  red.  During  this 
process  they  may  have  had  an  anticipatory  image  of  the 
blue  and  of  the  red.  But  when  the  red  came  they  did  not 
stop  to  call  up  memory  images  of  the  preceding  colors, 
and  decline  to  act  until  they  had  assured  themselves  that 
those  were  blue  and  white  instead  of  red.  Preparedness 
to  act  was  probably  already  secured  by  the  actual  occur- 
rence of  the  white  card  at  the  beginning  of  the  series.  In 
other  words,  while  images  may  have  been  present,  they 
were  images  with  a  future,  not  a  past  reference.  A  human 
being  reacting  to  a  series  of  stimuli  in  this  fashion  would 
but  rarely,  in  case  his  attention  had  wandered  during  the 
giving  of  the  first  two  stimuli,  have  to  recall  them  as 
memory  images  before  reaction,  but  he  might  very  likely 
have  anticipatory  images  of  the  stimuli  to  come  while 
waiting  for  them.  For  reasons  that  will  be  later  men- 


256  The  Animal  Mind 

tioned,  it  seems  probable  that  anticipation  rather  than  ret- 
rospection is  the  primitive  function  of  ideas. 

"It  may  still  be  objected,"  continues  Cole,  "that  retaining 
an  image  while  you  raise  three,  or  even  six,  colors  is  hardly 
retention  at  all,  so  short  is  the  time.  Of  course  the  fact  that 
the  animals  made  steady  and  rather  uniform  progress  for  six 
days  would  show  that  the  impression  was  not  effaced  in  twenty  - 
four  hours.  Number  one,  however,  was  given  a  review  of  his 
first  three-color  work  after  an  interval  of  eighteen  days.  He 
did  not  respond  to  the  three  blue  cards  at  all,  and  made  but 
one  mistake  in  twenty  trials  to  the  series  white,  orange,  blue, 
though  he  did  start  up  at  orange  six  times.  The  visual  images 
of  the  colors  must  therefore  have  been  retained  for  eighteen 
days  with  sufficient  clearness  to  permit  successful  responses." 
A  certain  confusion  of  thought  is  evident  in  this  paragraph. 
The  visual  images  of  course  were  not  retained  for  eighteen 
days;  what  was  retained  was,  possibly,  the  capacity  to  have 
the  visual  image  of  the  third  color  in  the  series  suggested  by 
the  actual  occurrence  of  the  second.  The  length  of  time 
this  capacity  persisted  is  quite  irrelevant  to  the  question  as  to 
whether  visual  ideas  were  really  present.  An  animal  incapable 
of  having  ideas  might  retain  the  effect  of  previous  stimulation 
for  a  long  period,  and  an  animal  that  had  ideas  might  lose  the 
power  of  having  a  particular  one  suggested  to  it  by  a  given 
stimulation  after  a  few  hours.  What  we  should  really  like  to 
know  is  whether  the  raccoons  could  think  of  color  number 
three  if  color  number  two  were  not  actually  shown  them  a 
few  seconds  earlier;  whether  they  could  "think  over"  the 
whole  performance  when  the  apparatus  was  not  there;  in 
short,  how  free  and  unhampered  by  the  control  of  present 
sense  stimulation  their  use  of  ideas  can  be.  Cole  concludes, 
"We  are  .  .  .  forced  to  believe  that  the  raccoon  retains  visual 
images."  We  are,  at  least,  shown  some  reason  for  thinking 


Modification  by  Experience  257 

that  memory  ideas  connected  with  immediately  preceding 
peripheral  stimulation  may  occur  in  the  raccoon's  mind. 

§  89.  Inhibition  involving   Discrimination  of  Simultaneous 

Stimuli 

Experiments  of  the  second  class,  where  the  different  stimuli 
are  simultaneously  presented,  have  been  made  by  Kinnaman 
on  monkeys,  by  Cole  on  raccoons,  by  Porter  and  Rouse  on 
birds,  and  by  Yerkes  on  the  dancing  mouse.  Kinnaman's 
Macacus  monkeys  entirely  failed  to  discriminate  cards  with 
different  figures  on  them  when  one  card  was  placed  on  a  box 
with  food  and  the  other  on  an  empty  box  (221).  The  English 
sparrow  and  the  cowbird,  on  the  other  hand,  both  learned  to 
do  this.  Monkeys  and  birds  alike  learned  to  discriminate 
glasses  covered  with  differently  colored  papers,  and  the  posi- 
tion or  number  of  a  vessel  in  a  series  (221,  345,  371).  Cole's 
raccoons  learned  to  discriminate  a  black  from  a  white  glass, 
and,  with  more  difficulty,  a  red  from  a  green  one  (82).  The 
monkeys  were  able  to  distinguish  fairly  well  differences  in 
size,  and  in  the  form  of  the  vessel.  The  birds  were  not 
tested  with  size  differences,  and  Porter's  birds  failed  to 
discriminate  the  vessel  forms;  Porter  suggests  that  the 
monkeys  may  have  been  helped  by  the  fact  that  the 
vessels  Kinnaman  used  differed  in  size  as  well  as  in  form. 
Rouse  found  that  his  pigeons  did  tolerably  well  in  learning 
form  differences  (371).  Our  own  experiments  with  the  chub, 
where  the  red  and  green  forks  were  presented  together  and  the 
fish  learned  rather  quickly  to  bite  at  the  red  rather  than  the 
green  even  when  both  were  empty  (421),  also  illustrate  this 
method,  as  does  the  case  of  the  chick  stung  by  the  bee,  who, 
on  the  basis  of  this  experience,  pecks  at  other  insects  but 
V  avoids  bees  (281).  Similarly,  Forel's  bees  and  wasps,  which 
were  trained  to  pick  out  pieces  of  paper  of  particular  colors 


258  The  Animal  Mind 

and  forms  because  they  have  been  previously  fed  from  them, 
exhibit  behavior  which  belongs  to  this  class  (130).  The 
method  was  used  also  by  Yerkes  in  the  experiments  to  test 
brightness,  color,  and  form  discrimination  in  the  dancing 
mouse,  described  on  pp.  145  and  197.  Yerkes  prefers  to 
establish  discrimination  by  associating  disagreeable  rather 
than  agreeable  experiences  with  one  of  the  alternatives, 
finding  that  the  motive  thus  constituted  works  with  greater 
uniformity.  Hence  his  mice  were  given  slight  electric  shocks 
when  they  made  the  "wrong"  choice,  instead  of  being  fed 
when  they  made  the  " right"  one.  He  describes  three  dif- 
ferent types  of  behavior  on  the  part  of  the  mice  in  making 
the  choices,  which  he  calls  choice  by  affirmation,  choice  by 
negation,  and  choice  by  comparison.  The  first  is  illustrated 
when  the  mouse  enters  the  right  compartment  at  once,  the 
second  when  it  goes  to  the  wrong  compartment  and  turns 
away  from  it,  the  third  when  it  vacillates  for  some  time  be- 
tween the  two  (469). 

§  90.   Comparison  of  Methods 

The  methods  just  described  have  something  in  common 
with,  and  something  different  from,  the  puzzle-box  method. 
In  both  cases  a  particular  object,  offering  certain  peculiarities 
to  the  senses,  and  distinguished  from  other  objects,  ultimately 
comes  to  occupy  the  focus  of  consciousness ;  but  in  this  method 
of  choice,  the  other  objects  are  themselves  connected,  either 
by  instinct  or  acquired  impulses,  with  a  particular  reaction 
which  has  to  be  checked.  No  definite  tendency  has  to  be  in- 
hibited in  the  puzzle-box  method;  it  is  only  necessary  for 
random  movements  to  be  dropped  off.  On  the  other  hand, 
these  experiments  where  inhibition  becomes  dependent  on 
the  presentation  of  a  particular  stimulus,  differ  from  tests 
like  those  on  the  hermit  crabs  or  the  water  scorpion,  in  that  the 


Modification  by  Experience  259 

latter  require  the  inhibition  of  an  entire  instinct.  The  case 
of  the  hermit  crab,  that  of  the  chub  presented  with  one  pair 
of  forceps  at  a  time,  and  that  of  the  chub  required  to  choose 
between  two  differently  colored  forceps,  may  represent  three 
lessening  degrees  of  the  amount  of  inhibitory  influence  exerted 
by  experience.  The  hermit  crab  entirely  abandoned  its 
ordinary  method  of  reacting  to  light.  The  whole  instinct 
vanished.  The  chub,  if  it  had  refused  as  a  result  of  expe- 
rience to  rise  to  the  green  fork  when  it  was  presented  alone, 
would  have  suspended  an  instinctive  action  so  far  as  that 
particular  stimulus  was  concerned,  and  would  have  been 
condemned  to  inactivity  simply  because  no  other  stimulus 
appealing  to  the  nutritive  instinct  presented  itself.  The  chub 
offered  a  choice  between  two  forks  is  not  required  to  suspend 
action  at  all,  save  for  the  brief  interval  necessary  to  discrimi- 
nate between  them.  We  should  naturally  expect  that  this 
third  state  of  affairs  would  be  the  easiest  to  bring  about,  and 
such  seems  to  be  the  case.  It  would  probably  be  effected  most 
quickly  when  one  of  the  stimuli  was  associated  with  positive 
pain,  instead  of  with  mere  absence  of  pleasure ;  hence,  very 
likely,  the  extremely  rapid  learning  of  Morgan's  chick. 
Obviously  the  difference  between  the  first  and  second  of  the 
two  cases  just  cited  is  at  bottom  one  of  degree,  not  of  kind. 
"A  whole  instinct"  means,  of  course,  a  reaction  to  a  whole 
class  of  stimuli ;  but  the  process  by  which  light,  for  instance, 
is  discriminated  from  other  forms  of  stimulation  cannot  be 
ultimately  different  from  the  process  by  which  one  kind  of 
light  stimulation  is  distinguished  from  another  kind. 

As  we  survey  the  processes  of  learning  involved  in  all  these 
methods,  the  labyrinth  method,  the  puzzle-box  method,  the 
method  of  inhibition,  and  the  method  of  inhibition  with  choice, 
we  find  that  they  are  all  cases  of  the  checking  of  movements 
which  do  not  involve  positively  pleasurable  results.  Their 


260  The  Animal  Mind 

psychological  aspect  in  every  case  means,  while  the  learning 
is  going  on,  the  diminution  of  unpleasantness  and  the  increase 
of  pleasantness ;  apart  from  this,  when  the  learning  is  com- 
pleted, it  differs  in  the  different  cases  as  regards  the  part  played 
by  the  consciousness  of  certain  more  or  less  accurately  dis- 
criminated objects.  As  the  learning  process  proceeds,  such 
objects,  as  we  have  seen,  come  to  stand  in  the  focus  of  atten- 
tion, so  that  to  the  cat  put  in  the  puzzle-box  the  string  that 
opens  the  door  is  instantly  attended  to ;  the  chick,  half  auto- 
matically pecking  at  various  objects  on  the  ground,  becomes 
vividly  conscious  of  the  appearance  of  a  bee  among  them ; 
the  monkey  becomes  aware  of  the  difference  in  color  between 
two  vessels  otherwise  quite  similar. 

§  91.    Visual  Memory  in  Homing 

Doubtless  all  the  phenomena  which  animals  exhibit  in  these 
various  experimental  tests  are  displayed  also  in  their  ordinary 
and  normal  life.  There  is  one  mode  of  behavior,  however, 
the  existence  of  which  has  been  established  by  careful  ob- 
servation of  an  animal  in  its  proper  environment,  that  does 
not  easily  find  an  analogue  among  the  facts  we  have  been 
describing.  I  refer  to  the  exercise  of  visual  memory  by  bees 
and  wasps.  The  case  of  the  bee,  indeed,  finding  its  way  from 
repeated  excursions  back  to  a  hive  which  remains  in  the  same 
place,  may  ultimately  involve  the  formation  of  a  habit  of 
movement  like  that  displayed  in  experiments  by  the  labyrinth 
method.  We  have  already  noted  on  pp.  138,  139  some  of  the 
evidence  that  bees  are,  at  least  in  their  earlier  flights  from  the 
hive,  guided  back  by  visual  memory.  Lubbock  found  that 
bees  from  a  hive  near  the  seashore,  when  taken  out  on  the 
water  and  liberated,  were  unable  to  find  their  way  home,  al- 
though the  distance  was  less  than  their  usual  range  of  flight 
on  land;  and  he  ascribes  their  failure  to  the  lack  of  visual 


Modification  by  Experience  261 

landmarks  to  guide  them  (248).  Bethe,  who  thinks  bees  are 
guided  home  neither  by  vision  nor  by  smell,  but  by  an  un- 
known force  to  which  they  respond  reflexly,  also  liberated 
some  bees  at  sea  about  1700-2000  metres  from  their  hive, 
which  was  near  the  foot  of  Vesuvius  and  beside  some  very  tall 
and  conspicuous  trees.  The  bees  failed  to  return,  yet  Bethe 
thinks,  if  they  were  guided  by  vision,  the  mountain  and  the 
trees  should  have  aided  them  to  do  so  (32).  It  may  well  be, 
of  course,  that  bees  cannot  see  objects  at  such  a  distance. 
Besides  his  observation  that  changing  the  appearance  of  a  hive 
did  not  disturb  the  bees  in  their  homing  flight,  Bethe  urges 
against  the  visual  memory  hypothesis  an  observation  on  a 
hive  which  had  on  one  side  of  it  a  garden,  and  on  the  other 
side  a  town,  which  he  thinks  the  bees  never  visited,  as  food 
was  to  be  had  in  abundance  in  the  garden.  Yet  when  lib- 
erated in  the  town  they  flew  back  to  the  hive  with  an  accuracy 
certainly  not  born  of  their  acquaintance  with  the  locality  (30). 
Von  Buttel-Reepen,  however,  doubts  whether  the  bees  really 
never  visited  the  town.  Bethe's  most  striking  illustration  of 
his  unknown  force,  however,  is  derived  from  his  "box-ex- 
periments." If  a  number  of  bees  are  carried  in  a  box  some 
distance  from  the  hive,  on  being  liberated  they  fly  straight 
up  in  the  air.  Some  of  them  will  return  to  the  hive,  but  if 
the  distance  is  great  enough,  many  will  drop  back  upon  the 
box.  Now  if  the  box  has  moved  only  a  few  centimeters 
away  during  the  flight  of  the  bees,  they  will  drop  back  to  the 
precise  spot  where  it  was,  and  take  no  notice  of  its  new  loca- 
tion. If  they  were  guided  by  vision,  Bethe  urges,  they  could 
easily  see  the  box  (30,  32).  This,  says  von  Buttel-Reepen,  is 
arguing  that  their  visual  memory  must  be  like  ours  if  it  exists 
at  all ;  it  may  be  a  memory,  not  of  the  appearance  of  the  box, 
but  of  its  locality.  He  himself,  repeating  Bethe's  experiments, 
observed  the  bees  on  dropping  back  after  their  upward  flight, 


262  The  Animal  Mind 

hunting  not  at  the  place  where  the  box  had  been,  but  at  a 
height  which  was  about  that  of  their  home-hive  entrance.  He 
thinks  that  an  important  feature  of  the  bee's  visual  memor} 
consists  in  a  power  of  accurately  estimating  height  above  the 
ground.  If  the  entrance  to  the  hive  be  raised  or  lowered  30 
cm.,  all  the  returning  bees  will  go  to  the  old  place,  and 
it  will  be  hours  and  sometimes  days  before  they  find  the 
new  one.  Moreover,  the  same  bees  tend  to  return  to  the  same 
corner  of  the  opening  each  time.  When  a  row  of  hives  had 
been  arranged,  some  with  openings  in  front  and  others  with 
openings  at  the  side,  bees  which  had  been  driven  home  ir 
haste  by  a  storm  would  sometimes  try  to  enter  the  wrong  hive 
but  if  their  home  hive  opened  on  the  side,  they  would  attempl 
to  enter  the  foreign  hive  on  the  corresponding  side  (72). 

It  may  be  granted  that  there  is  much  evidence  in  favoi 
of  the  use  of  visual  memory  by  bees,  although  the  differences 
which  must  exist  between  the  visual  perceptions  gained  by 
the  compound  eye  and  those  of  our  own  experience  neces- 
sarily complicate  the  phenomena  and  make  them  hard  tc 
interpret.  In  the  solitary  wasps,  although  Fabre  is  inclinec 
to  assume  a  "  special  faculty  "  of  homing,  independent  o: 
visual  memory,  basing  his  assumption  on  experiments  when 
the  wasps  returned  to  their  nests,  from  which  they  had  beer 
transported  in  a  box  to  a  distance  of  three  kilometers  (115 
Series  I) ;  yet  the  evidence  obtained  by  the  Peckhams  seem; 
fairly  conclusive  in  favor  of  memory  for  visual  landmarks 
The  solitary  wasps  have  been  shown  by  the  observations  o 
the  Peckhams  to  depend  upon  sight  for  the  return  to  th( 
nest  (322,  323),  and  the  same  conclusion  is  indicated  for  th< 
social  wasps  by  Enteman  (112).  The  Peckhams'  belief  ii 
the  visual  memory  of  solitary  wasps  rests  first  upon  the  fac 
that  the  wasp,  upon  completing  her  nest,  always  spend 
some  time  in  circling  about  the  locality,  in  and  out  amon] 


Modification  by  Experience  263 

the  plants,  as  if  she  were  making  a  careful  study  of  the 
region.  On  leaving  the  nest  a  second  time  she  omits  this 
process  and  flies  straight  away.  A  similar  "locality  sur- 
vey" is  made  by  hive  bees  and  by  social  wasps.  Secondly, 
the  Peckhams  argue  that  if  the  wasp  does  not  remember 
her  nest  by  landmarks,  it  ought  to  make  no  difference  to  her 
when  the  surroundings  are  altered  in  any  way.  They  found, 
however,  that  a  wasp  of  one  species  could  not  discover  her 
nest  when  a  leaf  that  covered  it  was  broken  off,  but  found 
it  again  without  trouble  when  the  leaf  was  replaced.  Another 
wasp  abandoned  the  nest  she  had  made  for  herself  with  much 
labor,  because  the  Peckhams,  to  identify  the  spot  themselves, 
drew  radiating  lines  from  it  in  the  dust.  A  third  argument 
against  the  existence  of  a  special  sense  of  direction  is  the  fact 
that  wasps  sometimes  are  unable  to  find  their  nests.  In  one 
case  the  Peckhams  dug  up  the  nest  of  a  wasp  and  she  made 
another  five  inches  away.  After  an  absence  of  three  hours  the 
wasp  returned,  and  seemed  to  be  puzzled  as  to  whether  the  old 
spot  or  the  new  one  were  the  place  of  her  nest.  "  At  first  she 
alighted  upon  the  first  site  and  scratched  away  a  little  earth, 
and  then  explored  several  other  places,  working  about  for 
twelve  minutes,  when  she  at  last  found  the  right  spot."  Simi- 
larly, when  a  wasp  that  was  carrying  her  prey  left  it  for  a  few 
moments  to  go  to  the  nest,  as  many  of  them  do,  apparently  to 
see  that  all  is  right  there,  if  any  of  the  surrounding  objects 
were  altered  she  often  had  great  difficulty  in  finding  the  prey 
again.  On  one  occasion  a  wasp  of  another  species  dug  its 
nest  in  the  midst  of  a  group  of  nests  of  the  Bembex  wasp. 
These  latter  are  usually  dug  in  a  wide  bare  space  of  earth 
which  has  no  vegetable  growth  to  serve  as  a  landmark.  When 
the  intruder  had  finished  her  nest,  it  looked  just  like  the  Bem- 
bex holes.  She  went  away,  secured  a  spider,  and  when  she 
returned  she  could  not  find  her  nest.  "  She  flew,  she  ran,  she 


264  The  Animal  Mind 

scurried  here  and  there,  but  she  had  utterly  lost  track  of  it. 
She  approached  it  several  times,  but  there  are  no  landmarks 
on  the  B.  field.  After  five  minutes  our  wasp  flew  back  to  look 
at  her  spider,"  which  she  had  dropped  about  three  feet  away, 
"  and  then  returned  to  her  search.  She  now  began  to  run  into 
the  B.  holes,  but  soon  came  out  again,  even  when  not  chased 
out  by  the  proprietor.  Suddenly  it  seemed  to  strike  her  that 
this  was  going  to  be  a  prolonged  affair,  and  that  her  treasure 
was  exposed  to  danger,  and  hurrying  back  she  dragged  it 
into  the  grass  at  the  edge  of  the  field,  where  it  was  hidden. 
Again  she  resumed  the  hunt,  flying  wildly  now  all  over  the 
field,  running  into  wrong  holes  and  even  kicking  out  earth 
as  though  she  thought  of  appropriating  them,  but  soon 
passing  on.  Once  more  she  became  anxious  about  the 
spider,  and,  carrying  it  up  on  to  a  plant,  suspended  it  there. 
Now  she  seemed  determined  to  take  possession  of  every 
hole  that  she  went  into,  digging  quite  persistently  in  each, 
but  then  giving  it  up.  One  in  particular  that  was  close  by 
the  spider  seemed  to  attract  her,  and  she  worked  at  it  so 
long  that  we  thought  she  had  adopted  it,  for  it  seemed  to 
be  unoccupied.  At  last,  however,  she  made  up  her  mind 
that  all  further  search  was  hopeless,  and  that  she  had 
better  begin  de  novo;  and  forty  minutes  from  the  time  that 
we  saw  her  first  she  started  a  new  nest  close  to  the  spider,  as 
though  she  would  run  no  more  risks"  (322).  An  occurrence 
of  this  kind  certainly  lends  color  to  the  '  recognition  of  land- 
marks '  theory.  On  the  other  hand,  the  Bembex  wasps  them- 
selves find  their  nests  with  unerring  accuracy,  though  there 
is  no  landmark  in  the  field.  Fabre  noted  that  Bembex  wasps 
could  not  be  led  astray  by  any  modification  of  either  the  look 
or  the  smell  of  their  nests,  and  thought  a  peculiar  form  of  space 
memory,  unparalleled  in  our  own  experience,  must  be  in- 
volved in  the  nest-finding  of  this  species  (115,  Series  I).  A 


Modification  by  Experience  265 

similar  kind  of  memory  for  pure  locality,  if  one  may  so  term 
it,  is  maintained  by  Goldsmith  to  exist  in  a  fish,  Gobius,  which 
lives  in  a  shell.  If  its  shell  habitation  is  moved  during  its 
absence,  the  fish  seeks  it  in  the  place  where  it  previously  was 
(146).  Bouvier,  repeating  Fabre's  experiments  on  Bembex, 
obtained  a  different  result.  When  a  stone,  for  example, 
that  had  been  at  the  mouth  of  a  Bembex  nest  was  moved 
a  distance  of  2  dm.,  the  wasp,  returning,  went  to  the 
stone.  Bouvier  accordingly  maintains  the  visual  landmark 
hypothesis  (68).  Ferton  holds  the  same  view  with  regard 
to  a  species  of  wasp  that  makes  its  nest  in  shells.  If  during 
successive  absences  on  the  wasp's  part  the  shell  is  moved  from 
position  A  to  position  B,  and  later  from  B  to  C  and  from  C  to 
D,  the  wasp,  returning,  goes  in  turn  to  each  of  the  positions 
that  the  shell  has  occupied.  "In  time,  she  omits  to  go  to  A, 
then  to  B.  Little  by  little,  the  image  of  the  previous  locations 
of  her  nest  is  effaced  in  the  insect's  memory."  When  she 
has  found  it,  after  each  displacement,  she  makes  a  new 
"locality  survey,"  before  starting  off  again  (116). 

The  performances  of  carrier  pigeons  in  finding  their  way 
home  have  been  the  subject  of  a  considerable  literature, 
and  many  theories  which  it  would  take  a  disproportionate 
amount  of  time  to  discuss.1  While  the  facts  are  not  easy  to 
explain,  careful  observations  on  young  pigeons  indicate  that 
their  powers  are  acquired,  not  innate,  and  that  they  are  in- 
fluenced by  visual  landmarks  (375).  What  guides  the  flight 
of  migrating  birds  over  vast  stretches  of  water,  or  the  young 
migrants  in  those  species  where  young  and  old  fly  by  different 
routes,  remains  a  mystery. 

If  we  take  the  case  of  the  solitary  wasp  as  typical  of  guid- 
ance by  visual  landmarks,  it  is  to  be  noted  that  no  gradual 

1  For  an  account  of  them  see  Claparede,  "  La  faculte  d'orientation  loin- 
taine  (Sens  de  direction,  sens  de  retour)  "  (76). 


266  The  Animal  Mind 

elimination  of  useless  movements  occurs,  as  in  many  species 
the  nest  is  revisited  but  once.  Nor  is  there  any  opportunity 
for  the  elimination  of  errors  through  their  unpleasant  conse- 
quences. Doubtless  the  wasp  does  not  choose  the  best  possi- 
ble route  for  her  return  to  the  nest,  and  doubtless  she  would 
improve  upon  it  if  she  made  repeated  journeys ;  but  at  least 
she  performs  at  the  first  trial  definite,  not  random,  responses 
to  stimuli  that  are  new  to  her ;  responses  that  are  not  wholly 
due  to  inherited  nervous  mechanism.  We  have  here  a  kind 
of  behavior  that  is  not  in  any  sense  "trial  and  error." 

Without  undertaking  the  difficult  task  of  explaining  it 
fully,  one  or  two  aspects  of  this  form  of  profiting  by  experience 
may  be  noted.  The  wasp,  when  she  has  finished  digging  her 
nest,  makes  a  "locality  survey" ;  that  is,  she  circles  about  the 
neighborhood  in  flight  for  a  few  minutes.  This  conduct  on 
her  part  is  doubtless  instinctive.  During  the  process  she 
receives  a  number  of  specific  visual  stimuli.  On  her  flight 
in  search  of  prey,  several  visual  landmarks  probably  impress 
her.  When  she  has  secured  her  spider  or  caterpillar,  she 
begins  the  return  flight.  We  cannot  attempt  to  explain  all 
the  mysteries  connected  with  this,  but  at  least  we  can  say  that 
the  flight  back  to  the  nest,  and  the  alighting  and  burying  the 
prey,  are  instinctive  actions  which  are  carried  out  only  under 
the  influences  of  the  same  visual  stimuli  that  the  animal  re- 
ceived on  its  locality  survey  and  its  outward  flight.  It  is 
essential  to  their  performance  that  the  wasp's  nervous  system 
should  receive  stimulation  like  that  which  it  has  received  a 
short  time  previously.  The  case  differs  from  the  formation 
of  a  habit,  such  as  we  saw  illustrated  in  the  labyrinth  experi- 
ments ;  for  while  in  a  habit  the  action  becomes  dependent  on 
a  certain  kind  of  stimulus  repeatedly  received,  here  it  is  not 
the  frequent  previous  experience  of  the  stimulus  that  renders 
it  and  it  alone  effective,  —  for  the  features  of  the  locality 


Modification  by  Experience  267 

may  have  been  quite  new  to  the  wasp  when  she  dug  her  nest 
there,  —  but  its  recent  previous  experience. 

The  great  thing  to  be  desired  with  regard  to  the  effect  of 
individual  experience  upon  behavior  is  that  it  shall  be  rapid. 
One  might  at  first  thought  say  also,  "that  it  shall  be  per- 
manent." But  it  is  probable  that  permanence  of  impression, 
though  valuable,  is  less  so  than  speed  of  modification;  for 
too  great  permanence  of  one  impression  would  interfere  with 
the  formation  of  new  ones.  We  have  been  often  told  of  the 
value  that  attaches  to  the  capacity  for  forgetting.  Judged 
by  /the  standard  of  rapidity,  the  guidance  of  action  by  a 
stimulus  that  has  been  experienced  only  once  before,  though 
recently,  is  a  form  of  modification  through  experience  dis- 
tinctly superior  to  the  guidance  of  action  by  repeated  stimu- 
lation. It  has  been  noted  already  that  if  the  effect  of  a  stimu- 
lus is  very  painful,  the  stimulus  does  not  need  to  be  repeated ; 
but  obviously  it  is  better  for  an  animal  to  modify  its  behavior 
rapidly  without  undergoing  pain,  which  must  tell  upon  its 
vital  energies. 

Furthermore,  we  may  notice  that  while  recency  of  experi- 
ence is  more  valuable  than  frequency  of  experience  as  regards 
the  rapidity  with  which  behavior  is  modified,  nothing  can 
take  the  place  of  frequency  where  permanence  is  concerned. 
It  is  not  to  be  desired  that  the  wasp  should  remain  perma- 
nently subject  to  the  influence  of  these  particular  landmarks. 
On  the  contrary,  their  influence  must  be  effaced,  in  order  that 
she  may  dig  and  stock  other  nests  in  other  localities.  The 
lasting  character  of  the  effect  produced  on  the  animals  in  the 
labyrinth  and  box  experiments  has,  on  the  other  hand,  been 
proved  by  nearly  all  experimenters.  One  of  Small's  rats 
opened  a  puzzle  box  with  speed  and  accuracy  after  an  inter- 
val of  forty  days,  during  which  time  she  had  had  no  tests 
whatever.  The  birds  tested  by  Porter  remembered  the  maze 


268  The  Animal  Mind 

well  after  an  interval  of  thirty  days,  and  one  of  Thorndike's 
monkeys  opened  a  puzzle  box  at  once,  eight  months  after  his 
last  previous  experience  with  it. 

Most  rapid  of  all  the  ways  in  which  conduct  may  be  modi- 
fied by  experience  is  the  method  of  the  memory  idea.  The 
wasp,  finding  her  way  back  to  her  nest,  is  guided  by  the  actual 
recurrence  of  certain  stimuli  which  she  has  experienced  on  her 
outward  flight.  As  she  has  needed  no  frequent  experience 
of  those  stimuli  to  make  them  effective,  her  behavior  is  far 
superior  to  that  of  the  frog  in  the  labyrinth.  But  if  she  pos- 
sessed the  power  to  direct  her  course  by  the  memory  image 
of  a  stimulus  rather  than  by  its  actual  recurrence,  still  more 
time  would  be  saved.  Suppose,  for  instance,  that  on  the 
devious  flight  in  search  of  prey  three  landmarks,  A,  B,  and 
C,  had  impressed  themselves  upon  the  wasp's  consciousness. 
Suppose  that  the  distance  in  a  straight  line  from  A  to  C  is 
less  than  the  distance  via  B.  Now  if,  on  the  return  trip, 
the  wasp  can  have  in  mind  not  only  C,  which  is  actually 
before  her,  but  the  memory  of  B  and  A  and  of  their  relative 
position,  she  can  greatly  shorten  her  course  by  flying  straight 
from  C  to  A.  Not  only  is  it  unnecessary  for  an  animal  capable 
of  memory  images  to  wait  for  the  repetition  of  stimuli  many 
times,  before  its  behavior  is  modified;  it  does  not  need  to 
wait  for  the  complete  repetition  of  a  series  of  stimuli  even  once, 
for  the  earlier  members  of  the  series  being  given,  the  later 
ones  are  suggested  in  idea. 

Yet  here,  too,  frequency  of  repetition  is  the  condition  of 
permanence.  I  think  it  may  be  said  that  if  an  animal  is 
capable  of  having  a  memory  image  at  all,  a  single  recent  ex- 
perience of  two  stimuli,  fully  attended  to,  is  enough  to  make 
one  of  them  call  up  the  image  of  the  other.  But  if  the  asso- 
ciation is  to  be  permanent,  nothing  but  frequent  repetition 
will  serve.  It  is  customary  to  say  that  a  single  very  vivid  ex- 


Modification  by  Experience  269 

perience  may  suffice  as  well  as  a  repeated  experience  to  pro- 
duce long  retention.  But  the  fact  is  that  a  single  vivid  ex- 
perience amounts  to  a  repeated  experience,  because  it  is 
usually  recalled  so  often  in  idea.  If  we  could  imagine  a 
person's  having  a  very  forcible  impression,  which  he  did  not 
once  recall  for  a  long  period  of  years,  it  is  doubtful  whether 
he  could  recall  it  at  the  end  of  that  time  with  anything  like 
the  success  which  attends  his  recollection  of  the  familiar  sur- 
roundings of  his  childhood. 


CHAPTER  XII 
THE  MEMORY  IDEA 

§  92.   Evidence  for  and  against  Ideas  in  Animals 

IN  the  last  chapter  we  have  seen  that  the  behavior  of  the 
lower  forms  of  animal  life,  at  least,  can  be  fully  explained 
without  supposing  that  the  animals  concerned  ever  consciously 
recall  the  effects  of  a  previously  experienced  stimulus  in  the 
entire  absence  of  the  stimulus  itself.  We  must  admit  that  it 
is  not  easy  to  prove  the  possession  by  any  animal  of  memory 
in  the  sense  of  having  ideas  of  absent  objects,  rather  than  in  the 
sense  of  behaving  differently  to  present  objects  because  of  past 
experience  with  them.  The  dog  shows  clearly  that  he  remem- 
bers his  master  in  the  latter  sense  by  displaying  joy  at  the 
sight  of  him.  Can  we  be  sure  that  he  has  remembered  him 
in  the  former  sense  during  his  absence ;  that  is,  that  he  has 
had  a  memory  image  of  him?  Certain  pieces  of  negative 
evidence  have  been  noted.  Where  an  animal  learns  to  work 
a  mechanism  by  gradually  dropping  off  unnecessary  move- 
ments, it  looks  as  if  its  conduct  were  not  guided  by  an  idea  of 
the  right  movements,  for  the  association  of  ideas  as  we  know 
it  is  so  rapid  a  process  that  a  single  experience  of  two  stimuli 
together  is  enough  to  enable  one  to  revive  the  other  in  the  form 
of  a  memory  idea,  provided  that  the  experience  was  recent. 
When  an  animal  hag  learned  to  run  through  a  complicated 
labyrinth  almost  without  error,  but  still  persists  in  taking  the 
wrong  turning  at  the  outset,  we  are  surely  justified  in  saying 
that  if  it  has  ideas,  it  does  not  use  them  as  a  human  being 
would,  for  some  kind  of  idea  of  the  right  way  to  start  the  laby- 

270 


The  Memory  Idea  271 

rinth  course  would  certainly  be  formed  in  a  human  mind  after 
a  very  few  experiences.  Thorndike's  attempts  to  make  cats 
and  dogs  learn  by  inferential  imitation,  and  by  putting  them 
through  the  movements  required,  while  they  do  not  show 
absence  of  ideas  in  the  animals'  minds,  indicate  that  ideas 
were  not  suggested  to  his  subjects  under  circumstances  which 
would  have  suggested  them  to  human  beings.  Cole's  op- 
posite results,  however,  weaken  Thorndike's  conclusions. 
Further,  the  way  in  which  instinctive  actions  are  often  per- 
formed by  animals  indicates  that  ideas  are  not  present  as  they 
would  be  to  a  human  being's  consciousness.  Human  beings 
do  some  things  from  instinct,  but  the  doing  of  them  may  be 
accompanied  by  ideas ;  a  mother's  care  for  her  child  involves 
ideas  of  the  child's  happiness  or  suffering,  and  of  its  future. 
;Enteman's  account  of  the  worker  wasp  which,  lacking  other 
'food  to  present  to  a  larva,  bit  off  a  portion  of  one  end  of  the 
larva's  body  and  offered  it  to  the  other  end  to  be  eaten,  jsug- 
gests  a  peculiar  limitation  of  ideas  in  the  wasp's  mi*ra,  at 
least  while  this  particular  function  was  being  performed  (112). 
rrhe  cow,  which  had  lamenteol  at  being  deprived  of  her  calf, 
and  on  having  the  stuffed  skin  of  her  offspring  given  to  her, 
licked  it  with  maternal  devotion  until  the  hay  stuffing  pro- 
truded, when  she  calmly  devoured  the  hay  (^79,  p.  334),  had 
perhaps  experienced  some  dim  ideas  connected  with  her  loss, 
but  certainly  her  consciousness  was  more  absorbed  by  the 
effects  of  present  stimulation  and  less  occupied  with  ideas 
than  a  human  mother's  would  have  been. 

On  the  other  hand,  certain  features  of  animal  behavior  are 
held  by  most  people  to  be  indications  that  the  creatures  thus 
acting  have  ideas  of  absent  objects.  Dogs  and  cats  are  sup- 
posed to  dream  because  they  snarl  and  twitch  their  muscles 
in  sleep ;  but,  as  Thorndike  has  pointed  out,  such  movements 
may  be  purely  reflex  and  unaccompanied  by  any  conscious- 


272  The  Animal  Mind 

ness  whatever.  A  dog  shows  depression  during  his  master's 
absence,  but  his  state  of  mind  may  be  merely  vague  discom- 
fort at  the  lack  of  an  accustomed  set  of  stimuli,  not  an  idea 
of  what  he  wants.  A  cat,  indeed,  once  observed  by  the 
writer,  did  behave  as  a  human  being  would  do  to  whom  an 
idea  had  occurred,  when,  on  coming  into  the  house  for  the 
first  time  after  she  had  moved  her  kittens  from  an  upper 
story  to  the  ground  floor,  she  started  upstairs  to  the  old  nest, 
stopped  half  way  up,  turned  and  ran  down  to  the  new  one. 
But  errors  of  interpretation  are  possible  at  every  turn  of  such 
observations.  An  attempt  was  made  by  Thorndike  to  test 
experimentally  the  presence  of  ideas  in  the  minds  of  the  cats 
he  was  studying  by  the  puzzle-box  method.  He  sat  near  the 
cage  where  the  cats  were  kept,  and  having  made  sure  that  the 
cats  were  looking  at  him,  he  would  clap  his  hands  and  say, 
"  I  must  feed  those  cats."  Aften  ten  seconds  he  would  take 
a  piece  of  fish,  go  to  the  cage  and  hold  it  through  the  wire 
netting;  the  cat,  of  course,  would  climb  up  and  get  the  food. 
After  from  thirty  to  sixty  trials  the  cat  learned  to  climb  up 
when  it  heard  him  clap  his  hands  and  speak,  without  waiting 
for  him  to  get  the  fish.  But  it  is  not  certain  that  the  hand- 
clapping  came  thus  to  suggest  to  the  cat  an  idea  of  the  ex- 
perimenter's taking  the  food  and  coming  to  the  cage ;  rather, 
in  the  course  of  so  many  repetitions,  the  clapping  of  the  hands 
may  have  become  a  direct  stimulus  to  the  act  of  climbing  up, 
although  Thorndike  thinks  that  the  ten  seconds7  interval  ren- 
dered this  improbable  (393).  Cole,  as  we  have  seen,  has 
observed  behavior  in  the  raccoon  that  might  well  be  regarded 
as  involving  ideas  (82). 

Despite  the  difficulty  of  proving  that  animals  have  memory 
ideas,  it  is  not  likely  that  any  such  gulf  separates  the  human 
mind  from  that  of  the  higher  animals  as  would  be  involved 
in  the  absence  from  the  latter  of  all  images  of  past  experiences. 


The  Memory  Idea  273 

That  ideas  occur  in  far  less  profusion  and  with  far  less  free- 
dom of  play  in  the  animal  mind  that  possesses  them  at  all 
than  in  the  human  mind ;  that  even  the  highest  animal  below 
man  lives  far  more  completely  absorbed  in  present  stimula- 
tions than  does  the  average  man,  seems  also  practically  cer- 
tain. In  the  lack  of  more  definite  knowledge  on  the  subject, 
we  may  discuss  a  few  related  questions  that  suggest  them- 
selves with  regard  to,  first,  the  primitive  function  of  ideas; 
secondly,  the  relation  of  ideas  to  qualitative  differences  in 
sensation;  and  thirdly,  the  nature  and  possible  origin  of 
"movement  ideas." 

§  93.    The  Primitive  Function  of  Ideas 

(i)  What  would  be  the  most  obvious  and  fundamental 
use  of  ideas  to  an  animal  ?  In  our  own  experience,  ideas  of 
absent  objects  have,  among  the  various  functions  they  sub- 
serve, two  that  are  rather  definitely  contrasted,  which  may  be 
termed  the  backward  and  the  forward  reference  of  ideas. 
On  the  one  hand,  we  recall  past  experiences  purely  as  such; 
we  indulge  in  "the  pleasures  of  memory,"  letting  the  at- 
tention wander  over  trains  of  ideas  recognized  as  belonging 
to  the  past.  On  the  other  hand,  we  form  ideas  of  experiences 
we  expect  to  have  in  the  future,  ideas  which  are  derived,  it 
is  true,  from  what  has  happened  in  the  past,  but  which  in- 
volve a  very  different  attitude  on  our  part  from  that  required 
by  mere  retrospection,  —  the  attitude,  namely,  of  anticipation, 
of  preparing  to  act  appropriately  to  the  situation  present  in 
idea.  Now  if  we  ask  which  of  these  two  functions  of  the  idea 
is  practically  the  more  important,  we  cannot  hesitate  to  say 
that  the  second  is.  To  recall  the  past,  except  for  the  pur- 
pose of  anticipating  the  future,  is  an  intellectual  luxury.  As 
Bentley  remarks,  "The  primary  use  of  the  image,  we  surmise, 
was  to  carry  the  organism  beyond  j;he  limits  of  the  immediate 


274  The  Animal  Mind 

environment,  and  to  assist  it  in  foreseeing  and  providing 
for  'the  future/  Its  function  seems,  then,  to  have  been  a 
prophetic  one;  it  was  a  means  to  what  we  may  term  remote 
adaptation.  .  .  .  The  past,  being  less  important  than  the 
future,  must  have  been  known  as  such  later  "  (23). 

It  is  in  making  possible  the  anticipation  of  a  coming 
stimulus,  thus  preparing  the  way  for  reaction,  that  the  mem- 
i  ory  image  is  most  fundamentally  useful.  Can  we  form  any 
conception  of  the  conditions  under  which  it  would  most 
naturally  make  its  appearance,  in  its  simplest,  most  rudi- 
mentary form  ?  Let  us  suppose  that  an  animal's  behavior  in 
a  certain  case  requires  a  definite  series  of  stimulations  for  its 
guidance.  The  acts  concerned  have  been  performed  several 
times,  so  that  when  the  reaction  to  number  one  in  the  series 
has  occurred,  the  motor  apparatus  concerned  in  the  reaction 
to  number  two  is  slightly  innervated,  although  the  actual 
giving  of  the  second  stimulus  is  necessary  to  produce  the 
movement.  Now  if  the  stimuli  follow  each  other  in  quick 
succession,  this  tendency  for  one  movement  to  help,  as  it 
were,  in  starting  the  next,  would  result  finally  in  the  perform- 
ance of  the  whole  set  of  reactions  "  automatically,"  with 
lapsing  consciousness.  But  suppose  the  sequence  is  slow, 
or  that  one  stimulus  in  the  series  is  delayed.  It  is  important, 
perhaps,  that  the  series  of  movements  shall  not  go  on  until 
the  delayed  stimulus  acts.  During  this  time  of  waiting,  it 
may  weH  be  that  the  nervous  energy  prepared  for  the  next 
reaction,  besides  innervating  to  some  degree  the  motor 
mechanism  that  will  be  needed,  overflows  into  the  sensory 
centres  which  the  anticipated  stimulus  is  to  stir  to  full  activity. 
The  result  for  consciousness  is  an  idea,  an  image,  though 
perhaps  rather  vague,  of  the  stimulus  waited  for.  Why,  it 
may  be  asked,  have  we  made  the  process  by  which  motor 
centres  become  "  associated,"  so  that  habits  are  formed,  and 


The  Memory  Idea  275 

the  innervation  of  one  centre  in  a  series  involved  in  successive 
reactions  produces  innervation  of  the  next,  fundamental, 
and  suggested  that  the  process  of  "  association "  whereby 
ideas  are  brought  into  consciousness  is  secondary  and  de- 
rived? Simply  because  we  find  the  formation  of  motor 
habits  far  down  in  the  animal  kingdom,  long  before  there  is 
any  evidence  of  the  existence  of  ideas.  It  is  interesting  to 
note  that  Judd  has  recently  advanced  the  theory  that  the  phys- 
iological process  underlying  the  ''association  of  ideas"  may 
involve  the  motor  pathways  (217).  In  any  case,  we  may  be 
pretty  sure  a  priori  that  the  primary  function  of  the  memory 
idea  or  image  is  to  anticipate  and  prepare  the  way  for  re- 
action to  a  coming  stimulus. 

§  94.    The  Significance  of  Stimuli  from  a  Distance 

(2)  Another  question  that  arises  in  connection  with  the 
origin  of  the  memory  idea  bears  on  the  possible  significance 
of  that  increase  in  ability  to  react  to  stimuli  from  a  distance 
which  we  find  characterizing  the  higher  animals.  An 
important  difference  must  exist  between  the  stimuli  from 
objects  directly  in  contact  with  an  organism's  body,  such  as 
give  rise  to  touch,  temperature,  taste,  and  pain  sensations  in 
our  own  experience,  and  those  which  proceed  from  objects  at 
a  distance,  such  as  forms  of  vibratory  energy  and  odors.  This 
difference  consists  in  the  fact  that  the  former  have  a  more 
direct  and  instant  effect  upon  the  organism's  welfare,  and  in 
consequence  demand  more  rapid  reaction,  than  the  latter. 
A  stimulus  in  immediate  contact  with  an  animal's  body  may 
have  a  harmful  or  a  beneficial  influence  at  the  moment  of  its 
impact ;  it  may  be  food  to  be  seized  or  an  enemy  to  be  es- 
caped, and  the  seizing  or  escaping  must  be  done  on  the  instant. 
On  the  other  hand,  if  an  animal  possesses  the  power,  belong- 
ing in  increasing  degree  to  the  higher  animals,  of  reacting  to 


276  The  Animal  Mind 

an  influence  proceeding  from  an  object  still  at  a  distance,  it 
becomes  safe  for  it  to  delay  the  reaction  after  the  stimulus  is 
given.  The  danger  is  not  so  imminent,  the  food  is  not  yet 
within  reach ;  the  full  motor  response  to  stimulation  may  be 
suspended  for  a  short  interval  without  imperilling  the  life 
interests  of  the  animal.,  Now  what  is  the  import  of  this 
delay  between  stimulus  and  reaction  for  the  memory  idea? 
It  seems  probable  that  the  reproduction  of  a  sensory  image 
by  central  excitation  demands  that  its  original  stimulus  shall 
have  left  upon  the  nervous  system  a  relatively  permanent 
effect.  We  may  distinguish  three  grades  of  animal  behavior 
in  response  to  stimulation.  First,  there  is  the  condition  where, 
so  far  as  we  can  see,  the  animal  does  not  learn  by  jndividual 
experience.  A  stimulus  entering  such  an  organism,  and 
sending  its  energy  out  again  through  whatever  motor  paths 
are  available,  leaves  so  little  effect  upon  the  substance  through 
which  it  passes  that  the  animal  behaves  toward  a  second 
stimulus  of  the  same  kind  precisely  as  it  did  toward  the  first. 
In  the  next  place,  we  have  the  grade  where  the  animal  learnsj 
by  experience,  without  having  the  power  to  recall  an  image  of 
its  experience.  The  chick  stung  by  a  bee  very  likely  cannot 
have  later  the  image  of  a  bee  suggested  to  him,  but  he  can  and 
does  refrain  from  picking  up  the  next  bee  he  sees.  Here  the 
stimulus  has  modified  the  behavior  of  the  animal,  and  has 
left  a  relatively  permanent  effect  of  some  sort  upon  the  ner- 
vous substance;  but  renewed  stimulation  from  without  is 
necessary  before  this  modification  makes  itself  apparent. 
Finally,  when  we  have  the  possibility  of  an  image,  purely 
centrally  excited,  and  not  leading  immediately  to  movement ; 
when  a  process  similar  to  the  original  may  be  set  up,  not  by 
an  influx  of  energy  from  without,  but  by  the  weaker  nervous 
current  coming  from  some  other  central  sensory  region,  it  is 
evident  that  the  nervous  substance  must  have  been  far  more 


The  Memory  Idea  277 

profoundly  affected  by  the  original  stimulus  than  it  was  in 
either  of  the  before-mentioned  cases.  What  characteristics 
of  a  stimulus  would  determine  how  strongly  and  deeply  it 
would  affect  the  nervous  substance  through  which  its  energy 
passed  ?  Its  intensity,  the  quantity  of  that  energy,  of  course ; 
but  still  more  emphatically  the  length  of  time  the  energy 
remained  in  the  centres  concerned,  without  being  drained 
off  into  motor  paths  and  transformed  into  bodily  movement. 
Not  merely  the  strength,  but  the  duration  of  the  current  de- 
termines how  deep  a  path  it  shall  dig  out  for  itself. 

Now,  as  we  have  seen,  stimuli  that  are  in  a  position  to  help 
or  harm  an  organism  at  the  instant  of  their  contact  with  its 
body  are  stimuli  demanding  immediate  motor  reaction.  In 
such  cases,  the  energy  of  the  stimulus  is  deflected  at  once 
into  the  appropriate  motor  path ;  it  is  not  delayed  long  enough 
in  the  sensory  regions  to  produce  any. permanent  change  there. 
But  where  the  animal  possesses  a  capacity  to  be  affected  by 
light  and  sound,  which  cannot  help  or  harm  at  the  moment 
of  their  action  upon  its  body,  then  reaction  may  be  postponed ; 
then  the  current  of  energy  sent  by  the  stimulus  into  the  ner- 
vous substance  is  not  at  once  drained  off,  but  may  linger 
sufficiently  long  to  produce  whatever  alteration,  whatever 
impress  upon  sensory  centres,  is  needful  to  insure  their 
subsequent  functioning  as  the  basis  of  a  memory  image. 
The  delay  between  stimulus  and  reaction,  made  possible  by 
sensitiveness  of  the  organism  to  stimuli  only  indirectly  affect- 
ing its  welfare,  may  then  supply  time  for  the  nervous  modi- 
fication to  be  produced  that  is  later  to  underlie  the  memory 
image,  as  the  delay  occupied  in  waiting  for  an  expected 
stimulus  offers  a  chance  to  bring  this  modification  into  play 
and  call  the  image  to  consciousness.  The  same  principle 
also  helps  to  explain  why  the  human  mind  gets  its  clearest 
and  most  controllable  memory  images  from  the  senses  whose 


278  The  Animal  Mind 

stimuli  do  not  indicate  direct  contact  of  a  beneficial  or  harm- 
ful object  with  the  body;  while  the  closer  and  more  direct 
the  stimulation,  as  for  instance  in  touch  and  organic  sensa- 
tions, the  obscurer  the  image.1 

Many  of  the  foregoing  sentences  are  taken  from  an  article 
by  the  writer  which  appeared  in  1904  (420).  A  very  interest- 
ing discussion  of  the  significance  from  the  neurological  stand- 
point of  reaction  to  stimulation  from  a  distance  is  to  be  found 
in  Sherrington's  recently  published  book  on  "The  Integrative 
Action  of  the  Nervous  System"  (382,  pp.  324  ff.).  Sher- 
rington  proposes  the  term  "distance  receptors"  for  those 
receptive  organs  "which  react  to  objects  at  a  distance,"  and 
declares  that  "the  distance  receptors  contribute  most\to  the 
uprearing  of  the  cerebrum."  The  -most  important  signifi- 
cance of  the  power  to  act  in  response  to  distant  objects 
Sherrington  finds  to  be  that  it  allows  an  interval  for  pre- 
paratory adjustment,  "for  preparatory  reactive  steps  which 
can  go  far  to  influence  the  success  of  attempt  either  to  obtain 
actual  contact  or  to  avoid  actual  contact  with  the  objec^." 
That  these  preparatory  steps  may  also  involve  the  germ  of 
the  memory  image  is  clearly  suggested  by  Sherrington. 
"We  may  suppose,"  he  says,  "that  in  the  time  run  through  by 
a  course  of  action  focussed  upon  a  final  consummatory  event, 
opportunity  is  given  for  instinct,  with  its  germ  of  memory, 
however  rudimentary,  and  its  germ  of  anticipation,  however 
slight,  to  evolve  under  selection  that  mental  extension  of  the 
present  backward  into  the  past  and  forward  into  the  future 
which  in  the  highest  animals  forms  the  prerogative  of  more 
developed  mind.  Nothing,  it  would  seem,  could  better 

1  An  exception  may  be  taken  to  this  statement  so  far  as  smells  are  con- 
cerned. Some  people  seem  to  have  difficulty  in  getting  memory  images  of 
odors.  For  the  writer,  such  images  are  among  the  most  vivid  and  most 
readily  controlled  in  her  experience. 


The  Memory  Idea  279 

insure  the  course  of  action  taken  in  that  interval  being  the 
right  one  than  memory  and  anticipatory  forecast."  The 
present  writer's  views  regarding  the  significance  of  the  delay 
made  possible  by  reaction  through  "distance  receptors," 
while  independently  formed,  find  thus  most  valuable  support. 

§  95.    Ideas  of  Movement 

(3)  A  very  striking  difference  between  man  and  most  of 
the  lower  animals  lies  in  the  immensely  greater  number  of 
different  movements,  each  adapted  to  some  feature  of  the 
environment,  that  man  is  able  to  perform.  When  we  think 
of  the  enormous  variety  of  muscular  adjustments  of  which  the 
human  race  as  a  whole  is  capable,  and  compare  it  with  the 
limited  power  of  an  earthworm  to  react  upon  its  surround- 
ings, the  small  extent  of  its  motor  repertoire,  the  gulf  that 
separates  them  is  highly  impressive.  And  the  conscious 
experience  of  an  animal  must  be  profoundly  modified  by  the 
number  and  variety  of  the  motor  coordinations  it  has  under  its 
control ;  not  only  because  sensory  discriminations  in  general 
involve  differentiation  of  motor  reaction,  but  because  that 
breaking  up  of  the  crude  mass  of  sense  impressions  into 
smaller  masses  which  we  call  the  perception  of  external 
objects  depends  so  largely  on  what  the  animal  is  able  to  do 
with  objects.  Think,  for  example,  of  a  creature  able  to 
move  in  response  to  its  environment,  but  not  able  to  alter 
the  relative  position  of  different  features  of  that  environment ; 
not  able,  in  plain  words,  to  pick  up  a  single  object  and  move 
it  about.  " Objects"  for  such  an  animal  simply  would  not 
exist.  There  would  be  a  vague  background  of  sensation 
qualities,  but  no  sharply  defined  groups  of  such  qualities. 
An  object  to  the  human  mind  is  essentially  a  bit  of  expe- 
rience with  which  things  can  be  done ;  which  can  be  moved 
about  independently  of  its  surroundings,  "handled,"  used 


280  The  Animal  Mind 

for  one  purpose  or  another.  The  perception  of  objects 
as  distinct  entities  increases  with  the  power  of  making  defi- 
nitely coordinated  and  adjusted  motor  responses  to  them. 
That  one  important  condition  to  the  production  of  such  re- 
sponses lies  in  the  possession  of  a  grasping  organ,  a  highly 
movable  member  that  can  seize  objects  firmly  and  thus  move 
them  about,  is  self-evident.  The  elephant  and  the  monkey, 
which. possess  such  organs,  must  have  far  more  definite  per- 
ceptions of  objects,  as  individual  entities  to  be  separated 
from  their  backgrounds  and  used,  than  any  other  lower 
animals.  But  to  the  acquisition  of  the  most  complicated  and 
perfect  systems  of  motor  reactions  another  factor  contributes. 
This  factor  is  the  movement  idea.  A  movement  idea  is 
the  revival,  through  central  excitation,  of  the  sensations, 
visual,  tactile,  kinaesthetic,  originally  produced  by  the  per- 
formance of  the  movement  itself.  And  when  such  an  idea 
is  attended  to,  when,  in  popular  language,  we  think  hard 
enough  of  how  the  movement  would  "feel"  and  look  if  it 
were  performed,  then,  so  close  is  the  connection  between 
sensory  and  motor  processes,  the  movement  is  instituted 
afresh.  The  movement  is  willed  by  attending  to  the  idea  of 
it.  This  is  the  familiar  doctrine  expounded  by  James  in 
Chapter  XXVI  of  his  "  Psychology  "  (189).  Recently  it  has 
been  pointed  out  that  the  "willing"  of  a  movement  by  no 
means  always  or  even  usually  involves  preliminary  attention 
to  a  movement  idea  (e.g.  445).  This  is  undoubtedly  true. 
Nearly  or  quite  all  the  movements  executed  by  a  man  in  the 
ordinary  course  of  a  day  are  movements  that  he  has  made 
many,  many  times  before.  And  movements  that  have  been 
repeatedly  made  come  to  be  made  in  response  to  stimuli 
that  through  association  have  been  substituted  for  the  origi- 
nal processes  inducing  movement.  When  I  see  my  handker- 
chief on  the  floor  I  do  not  need  to  think  beforehand  of  what 


The  Memory  Idea  281 

stooping  and  picking  it  up  will  feel  like;  the  sight  of  that 
object  in  that  position  sets  off  the  appropriate  movement 
directly.  When  the  soldier  hears  the  command  "Halt!" 
he  does  not  first  think  of  stopping;  the  sound  stops  him. 
But  the  important  consideration  is  not  what  conditions  de- 
termine old  movements,  movements  that  have  been  many 
times  performed  by  the  individual.  The  superiority  of  an 
animal  consists  largely  in  its  power  to  learn  new  movements 
rapidly.  And  whenever  we  ourselves  learn  really  new 
movements,  we  find  that  an  essential  part  of  the  process  is 
the  presence  of  a  movement  idea  in  the  focus  of  attention. 

Such  processes  as  those  involved  in  learning  the  type- 
writer, in  learning  to  play  golf,  in  acquiring  any  new  set  of 
muscular  adjustments,  certainly  involve  calling  up  in  the 
form  of  ideas  the  sensory  experiences  obtained  from  actually 
moving.  We  have  to  " think"  where  the  fingers  must  go, 
how  the  arms  must  swing ;  the  trainer  who  instructs  us  puts 
forth  every  effort  to  suggest  to  us  the  proper  look  and  feel  of 
the  movements  themselves.  He  must,  of  course,  in  so  doing 
recall  to  us  the  ideas  of  the  movements  already  familiar  to 
us  which  are  most  nearly  similar  to  the  required  new  ones. 
Where  nothing  similar  can  be  found,  the  training  is  likely  to 
fail.  The  difficulty  experienced  by  an  average  human  being 
in  learning  to  move  his  ears  consists  essentially  in  the  fact  that, 
never  having  done  anything  remotely  similar  to  moving  his 
ears,  he  has  no  movement  idea  to  call  up.  He  cannot  move 
them  because  he  cannot  "imagine"  how  it  would  feel  to 
move  them. 

Thus  the  power  to  attend  to  a  memory  idea  of  the  sensa- 
tions formerly  involved  in  the  performance  of  a  movement  is 
a  very  important  factor  in  the  rapid  acquisition  of  new  move- 
ments. And  one  reason  why  the  lower  animals  in  general 
learn  new  movements  but  slowly  may  be  connected  with  a 


282  The  Animal  Mind 

lack  of  development  of  the  power  to  attend  to  movement 
ideas.  For  the  slight  development  of  this  power  in  most  of 
the  lower  animals  there  is  at  least  one  obvious  reason.  The 
life  of  an  animal  in  natural  conditions  demands  that  its 
attention  shall  be  constantly  directed  outward.  It  is  en- 
gaged in  continual  watchfulness  for  food  and  enemies.  The 
stimuli  which  come  to  it  from  external  objects  demand  all 
its  mental  energies ;  the  successful  animal  is  the  wide-awake, 
alert  animal.  How  can  it,  with  every  available  avenue  of 
sense  wide  open  to  the  external  world,  with  every  unit  of 
mental  capital  invested  in  watching  and  listening  and  smell- 
ing, spare  any  mental  energy  to  attend  to  the  sensations  from 
its  own  movements  ?  It  sees  the  prey,  it  makes  an  elaborate 
series  of  movements  in  response  to  the  sight ;  but  if  it  were  to 
attend  for  one  instant  to  the  sensations  from  the  movements 
themselves,  there  would  be  a  relaxation  of  its  watchfulness 
of  external  things  that  might  mean  the  escape  of  the  prey. 
But  unless  it  attends  to  the  sensations  resulting  from  move- 
ment, it  will  not  reproduce  them  in  idea.  That  which  is 
unattended  to  when  originally  experienced  is  ordinarily  not 
recalled. 

It  would  thus  seem  as  though  one  condition  which  must  be 
fulfilled  if  movement  ideas  are  to  play  an  important  part  in  a 
creature's  experience  were  that  the  animal  should,  for  a  time 
at  least,  be  set  free  from  the  pressure  of  the  practical  hand-to- 
hand  struggle  for  the  means  of  existence,  and  thus  enabled 
in  safety  to  attend  to  its  own  movement  sensations.  Animal 
play,  at  first  thought,  offers  an  instance  of  such  liberation 
from  practical  necessities.  But  as  Groos  has  shown,  animal 
play  is  not  so  unpractical  as  it  looks  (154).  It  is  simply  the 
exercise  of  the  same  instincts  upon  which  in  other  circum- 
stances the  animal's  welfare  depends.  The  attention  is 
absorbed  in  external  objects  quite  as  much  in  play  as  in 


The  Memory  Idea  283 

the  actual  chase  or  warfare.  The  kitten  watches  the  string, 
for  which  she  has  no  practical  use,  as  intently  as  she  watches 
the  bird  for  which  she  does  have  a  practical  use;  the  dogs 
rolling  over  and  over  each  other  are  nearly  as  absorbed  in  each 
other's  movements  as  if  they  were  in  deadly  combat. 

That  relief  from  practical  necessity  which  will  serve  the 
purpose  we  are  considering  is  to  be  found  not  in  play,  but 
in  infancy.  If  a  creature  spends  the  period  during  which 
its  nervous  system  is  undergoing  most  rapid  development 
in  a  state  of  complete  shelter  and  protection  from  external 
danger,  with  all  its  vital  needs  supplied,  then  the  nervous 
energy  which  under  other  conditions  would  be  expended  in 
the  processes  underlying  attention  to  external  stimuli  is 
free  to  be  so  devoted  that  attention  will  be  directed  toward 
the  creature's  inner  experiences.  The  human  baby,  while 
he  may  be  interested  in  lights  and  sounds,  in  external  impres- 
sions, does  not  need  to  be  alert  and  watchful  lest  he  miss  his 
dinner  or  be  dined  on  himself;  his  attention  is  free  to  be 
expended  on  his  own  movement  experiences  as  well  as  on 
anything  else.  That  young  children  do  go  through  a  stage 
of  intense  interest  in  the  sensations  resulting  from  their  own 
movements  is  a  fact  made  clear  from  many  observations. 
The  curious  period  of  "self-imitation"  in  the  child  when  it 
repeats  for  an  indefinite  period  the  same  movement  or  sound,, 
over  and  over  again  (8),  is  very  likely  a  period  of  vivid 
attention  to  movement  sensations ;  and  just  as  the  movement 
will  take  place  if  we  attend  exclusively  to  the  idea  of  it,  so 
here  the  child's  developed  attention  to  the  sensations  result- 
ing from  the  movement  reinstates  the  movement  itself. 

That  the  prolonged  period  of  human  infancy  is  of  advan- 
tage to  the  intellectual  life  of  man  because  it  means  plasticity, 
the  absence  of  fixed  instincts  that  would  take  the  place  of 
acquisition  by  individual  experience,  was  first  pointed  out  by 


284  The  Animal  Mind 

Fiske  (126).  But  quite  as  important  is  the  fact  that  in  pro- 
longed infancy  we  have  the  opportunity  for  acquiring  the 
habit  of  that  attention  to  our  own  movements  which  is  the 
prerequisite  for  the  movement  idea.  There  are,  as  we  have 
seen,  various  ways  of  learning  by  experience  —  slow  ways 
that  do  not  involve  ideas,  and  the  rapid  way  that  does.  The 
great  advantage  of  man  over  most  of  the  lower  animals  is  not 
so  much  in  the  fact  as  in  the  method  of  his  learning.  One 
of  the  most  vital  meanings  of  the  long  period  of  helplessness 
and  dependence  constituting  human  infancy  lies  in  the  fact 
that  by  relieving  from  the  necessity  of  attending  exclusively 
to  external  objects,  it  renders  possible  attention  to  the  sensa- 
tions resulting  from  movement;  and  thus,  by  supplying  an 
essential  condition  for  the  revival  of  such  sensations  in  idea, 
it  opens  the  way  for  the  control  of  movement  through  the 
movement  idea. 


CHAPTER  XIII 
SOME  ASPECTS  OF  ATTENTION 

THE  student  absorbed  in  reading  "does  not  hear"  an  ap- 
proaching footstep.  That  is,  a  stimulus  which  would  under 
other  circumstances  produce  an  effect  loses  a  great  part  of 
its  influence  because  of  the  fact  that  another  stimulus  is  al- 
ready upon  the  field.  This  other  stimulus  need  not  be  more 
intense,  that  is,  need  not  involve  more  physical  energy, 
than  the  one  which  is  gnored.  It  does  not  win  the  victory 
by  a  mere  swamping  of  its  rival  through  its  superior  quantity. 
A  man  may  walk  along  city  streets,  his  eyes  and  ears  bom- 
barded with  brilliant  lights  and  loud  sounds,  and  yet  the 
centre  of  his  consciousness  may  be  a  train  of  ideas,  repre- 
senting in  their  physical  accompaniment  in  his  cortex  a 
quantity  of  energy  insignificant  compared  with  that  of  the 
external  stimuli  pouring  in  upon  him.  Psychologists  com- 
monly express  this  fact  by  saying  that  while  the  strength  of 
a  stimulus  conditions  the  intensity  of  the  mental  process  ac- 
companying it,  the  clearness  of  that  process  depends  upon 
attention. 

§  96.    The  Interference  of  Stimuli 

Attention,  then,  is  the  name  given  to  a  device,  whatever 
its  nature,  whereby  one  stimulus  has  its  effectiveness  in- 
creased over  that  of  another  whose  physical  energy 
may  be  greater.  What  happens  in  the  simpler  forms  of 
animal  life  when  two  stimuli,  requiring  different  reactions, 
operate  simultaneously?  We  may  quote  from  Jennings  the 


286  The  Animal  Mind 

facts  about  Paramecium.  "If  the  animal  is  at  rest  against 
a  mass  of  vegetable  matter  or  a  bit  of  paper,  .  .  .  and  it 
is  then  struck  with  the  tip  of  a  glass  rod,  we  find  that  at  first 
it  may  not  react  to  the  latter  stimulus  at  all."  "A  strong 
blow  on  the  anterior  end  causes  the  animal  to  leave  the  solid 
and  give  the  typical  avoiding  reaction."  "If  specimens 
showing  the  contact  reaction  are  heated,  it  is  found  that  they 
do  not  react  to  the  heat  until  a  higher  temperature  is  reached 
than  that  necessary  to  cause  a  definite  reaction  in  free-swim- 
ming specimens."  "On  the  other  hand,  both  heat  and  cold 
interfere  with  the  contact  reaction.  Paramecia  much  above 
or  much  below  the  usual  temperature  do  not  settle  against 
solids  with  which  they  come  in  contact,  but  respond  in- 
stead by  a  pronounced  avoiding  reaction."  "Specimens  in 
contact  with  a  solid  react  less  readily  to  chemicals  than  do  free 
specimens.  .  .  .  On  the  other  hand,  immersion  in  strong 
chemicals  prevents  the  positive  contact  reaction."  "The 
contact  reaction  may  completely  prevent  the  reaction  to 
gravity,"  and  to  water  currents.  It  also  modifies  the  reaction 
to  the  electric  current.  While  a  part  of  the  influence  exerted 
by  the  contact  reaction  on  other  responses  may  be  purely 
physical,  due  to  the  fact  that  an  actual  secretion  of  mucus 
may  occur  whereby  the  animal  "sticks  fast"  to  the  solid, 
yet  this  alone  does  not  explain  the  facts,  for  the  cilia  that  are 
not  attached  do  not  behave  normally.  The  reaction  to  gravity 
regularly  yields  whenever  opposed  to  the  action  of  any  other 
stimulus  (211,  pp.  92  ff.). 

Sometimes  the  action  of  one  form  of  stimulation  merely 
affects  the  form  of  the  response  to  another,  as  in  the  case  where 
abnormal  temperature  causes  the  avoiding  instead  of  the 
positive  reaction  to  be  given  to  solids.  In  other  cases,  re- 
action to  one  of  the  stimuli  is  suppressed  or  weakened.  The 
facts  suggest  that  the  influential  stimulus  is  either  the  ont 


Some  Aspects  of  Attention  287 

that  is  on  the  field  first  (the  contact  reaction  may  prevent  re- 
sponse to  temperature,  or  abnormal  temperature  may  modify 
the  contact  reaction),  or  the  one  that  is  the  more  important 
(gravity  yields  always  to  other  stimuli). 

In  some  higher  animals  the  effects  of  interference  of  stimuli 
have  been  noted.  The  earthworm  will  not  respond  to  light 
if  feeding  (91)  or  mating  (179).  In  the  turbellarian  Con- 
voluta  roscoffensis  light  is  victorious  over  heat  in  determining 
reaction.  The  animals  in  their  positively  phototropic  phase 
will  remain  in  the  heated  light  end  of  a  vessel  until  they  perish. 
Light  and  gravity  are  more  nearly  balanced  in  their  effects. 
Convoluta  is  negatively  geotropic,  yet  if  the  brightest  region 
is  below  the  surface,  the  animals  will  go  there.  But  if  this 
region  is  only  a  little  brighter  than  the  surface,  they  will 
stay  at  the  surface,  gravity  dominating  (140).  The  sea 
urchin  shows  in  its  behavior  a  somewhat  similar  relation 
between  mechanical  and  chemical  stimulation.  If  weak 
acid  is  dropped  into  the  water  containing  specimens  of  Arba- 
cia,  their  spines  begin  to  interlace.  A  slight  shaking  will 
restore  them  to  the  normal  position,  but  if  more  acid  be 
added,  no  mechanical  stimulation  will  overcome  the  effect 
of  the  chemical  (409).  Various  facts  concerning  the  inter- 
relations- of  gravity  and  light  as  stimuli  have  been  noted  in 
Chapter  IX.  A  very  interesting  case  of  the  suppression  of 
one  reaction  by  another  is  reported  by  Holmes  in  his  obser- 
vations on  the  water  insect  Ranatra.  The  positive  response 
of  this  insect  to  light,  very  precise  and  striking,  may  be 
wholly  suspended  when  the  animal  is  feeding,  when  a  num- 
ber of  individuals  are  collected,  when  the  insect  stops  to 
clean  itself,  or  even  "by  the  sudden  appearance  of  a  large 
object  in  the  field  of  vision,"  behavior  which  is  strongly 
suggestive  of  the  "distraction  of  attention"  in  a  human 
being  (186).  Roubaud,  in  a  study  of  the  behavior  of  some 


288  The  Animal  Mind 

species  of  flies  that  live  on  the  seashore,  feeding  on  dead 
fish  and  the  like,  says  that  they  will  abandon  the  " head-on" 
position  which  they  regularly  assume  toward  the  wind,  if 
attracted  by  the  odor  of  food  (370). 

Wherever  we  find  that  one  class  of  stimuli  regularly 
yields  to  another  if  the  two  act  together,  it  is  safe  to  assume 
that  the  prepotent  stimulus  is  more  important  to  the  organ- 
ism's welfare  than  the  vanquished  one.  And  while  we  can- 
not without  more  ado  call  such  cases  of  the  interference  of 
stimuli  as  are  found  in  very  simple  animals  cases  of  attention, 
and  ascribe  to  their  psychic  accompaniments  all  the  character- 
istics of  attention  as  a  feature  of  our  own  experience,  yet  we 
may  assert  that  they  have  in  common  with  attention  the  sig- 
nificance of  being  a  device  to  secure  reaction  to  the  most  vitally 
important  of  several  stimuli  acting  at  once  upon  the  organism. 

§  97.  Methods  of  securing  Prepotency  of  vitally  Important 

Stimuli 

An  inanimate  object  acted  upon  by  several  forces  at  once 
is  determined  in  its  motion  by  their  relative  intensity.  Con- 
ceivably, an  extremely  simple  form  of  animal  life,  when 
subjected  to  two  stimulations  acting  together,  would  also 
respond  in  a  way  answering  precisely  to  the  relative  strength 
of  the  two.  It  is  easy  to  see  what  would  be  the  disadvantage 
of  such  a  state  of  affairs  for  the  animal.  The  weaker  of 
the  two  stimuli  might  be  of  far  greater  significance  for  organic 
welfare  than  the  stronger.  For  example,  it  would  often  be  im- 
portant that  an  animal  should  be  able  to  respond  to  a  very 
faint  food  stimulus  rather  than  to  any  of  the  stronger  forces 
acting  upon  it.  Evidently  a  prime  need  of  animal  life  is 
some  arrangement  whereby  weak  but  important  stimuli 
shall  be  given  the  preference  in  determining  reaction  over 
stronger  but  less  vitally  necessary  ones.  Sense  organs  are 


Some  Aspects  of  Attention  289 

one  such  device.  The  comparatively  slight  amount  of 
chemical  energy  coming  from  a  bit  of  food  may  have  its 
effectiveness  for  the  nervous  system  greatly  increased  through 
its  reception  by  a  structure  adapted  to  use  the  whole  of  it 
to  advantage.  Light  stimulation  involves  a  quantity  of 
energy  that  is  insignificant  in  comparison  with  tljgLjgrosser 
forces  acting  on  an  organism;  yet  falling  on  the  retmaT^the- 
energy  is  economized  and  magnified  through  the  stored--up 
chemical  forces  it  sets  free.  Thus  a  weak  stimulus  may  by 
a  sense  organ  be  made  powerful  to  determine  reaction. 
Another  arrangement  to  the  same  effect  is  the  peculiarity 
of  the  nervous  system  whereby,  through  an  arrangement 
akin  to  the  summation  of  faint  stimuli,  a  moving  stimulus, 
one  acting  successively  upon  neighbor  ng  points  of  a  sensitive 
surface,  produces  an  effect  disproportionate  to  its  intensity. 
A  moving  stimulus  is  a  vitally  important  stimulus ;  it  means 
life,  and  hence  may  mean  food  or  danger.  The  response  to 
it  is  in  most  cases  adapted  rather  to  its  importance  than  to 
its  physical  strength  A  third  arrangement  for  the  securing 
of  reaction  to  vitally  important  stimulation  lies  in  the  existence 
of  preformed  connections  in  the  nervous  system,  which  bring 
it  about  that  the  path  of  the  excitation  produced  by  one  stimu- 
lus is  clear  to  the  motor  apparatus,  while  that  of  another  is 
closed.  Reactions  of  this  sort  we  call  instinctive.  The 
nesting  bird  responds  to  the  sight  of  building  material  rather 
than  to  that  of  objects  offering  equally  strong  stimulation 
to  the  optic  nerve ;  the  cat  sits  at  the  mouse  hole,  the  parent 
animal  responds  to  the  faintest  cry  of  the  offspring,  because 
these  stimuli  have  the  right  of  way  by  virtue  of  inherited 
nervous  connections. 

Finally,  a  weak  stimulus  may  determine  reaction  and  be 
victorious  over  a  stronger  one  because  of  nervous  pathways 
formed  throught  he  individual's  own  experience.  The  conse- 


290  The  Animal  Mind 

quences  of  reaction  to  it  in  the  individual's  past  may  operate 
to  secure  reaction  to  it  in  the  future.  To  the  cat  in  a  puz- 
zle box,  the  string  that  must  be  pulled  to  let  it  out  offered 
originally  no  stronger  stimulus  to  action  than  any  other  ob- 
ject in  sight ;  but  after  sufficient  experience  the  string  comes 
to  dominate  the  situation  and  determine  the  cat's  behavior. 
If  the  experience  of  consequences  is  slowly  acquired,  by 
many  repetitions,  the  process  of  reacting  to  an  object  originally 
indifferent  may  be  unaccompanied  by  any  ideas  of  the  con- 
sequences of  such  reaction.  If  it  is  rapidly  acquired,  we 
know  that  we  human  beings  at  least  accompany  our  reactions 
by  calling  up  the  results  of  our  past  reactions  in  the  form  of 
memory  ideas. 

§  98.    The  Peculiar  Characteristics  of  Attention  as  a  Device 
to  secure  Prepotency 

We  have  suggested  that  attention  is  a  means  of  securing 
reaction  to  the  vitally  important  stimuli  acting  upon  an 
organism.  Does  reaction  to  a  stimulus  always  mean  atten- 
tion to  the  sensation  accompanying  that  stimulus  ? 

This  question  may  best  be  answered  by  examining  the 
characteristics  of  the  attention  process  as  we  know  it.  In 
attention,  the  details  of  the  object  attended  to  become  clear 
and  distinct.  That  is,  attention  is  a  state  where  discrimina- 
tion is  improved.  Further,  attention  involves  varying  de- 
grees of  effort,  and  these  are  marked  by  varying  intensity  of 
certain  bodily  processes.  Attention  under  difficulties  is 
accompanied  by  a  rigid  position  of  the  body,  by  holding  the 
breath,  and  by  various  muscular  effects,  aside  from  the  pro- 
cesses which,  like  frowning,  are  concerned  with  the  adaptation 
of  the  sense  organ  to  receive  an  impression.  These  general 
bodily  effects  of  attention  are  all  such  as  to  suggest  that  the 
body  is  to  be  kept  as  quiet  as  possible  during  the  attentive 


Some  Aspects  of  Attention  291 

state.  In  other  words,  no  reaction  is  to  be  made  to  the 
object  attended  to  except  such  as  may  be  necessary  to  allow 
its  being  carefully  discriminated  from  other  objects.  At- 
tention, in  its  intenser  degrees,  at  least,  seems  to  involve  a 
state  of  suspended  reaction. 

Not  every  case,  then,  of  response  adapted  to  the  vital  im- 
portance of  a  stimulus  is  a  case  that  suggests  as  its  psychic 
aspect  attention  to  the  accompanying  sensation.  When,  for 
example,  a  reaction  of  especial  speed  is  made  to  contact  with 
a  moving  stimulus,  the  speed  of  the  reaction  would  itself 
indicate  that  the  sensations  produced  are  not  attended  to. 
The  proper  situation  for  attention  would  be  the  situation  in 
which  the  reaction  needs  to  be  suspended  until  the  stimu- 
lus is  fully  discriminated.  Now  such  careful  discrimination 
does  not  appear  to  be  characteristic  of  reactions  that  are 
largely  based  on  inherited  nervous  structures.  Many  facts 
concerning  the  instincts  of  animals,  that  is,  their  inherited 
reactions,  indicate  that  these  are  extremely  rough  adjust- 
ments of  behavior  to  environment  until  refined  by  individual 
experience.  Hudson  observed,  for  example,  that  newly  born 
lambs  on  the  South  American  plains  had  a  tendency  to  run 
away  from  any  object  that  approached  them,  and  to  follow 
any  object  that  receded  from  them.  They  would  follow 
his  horse  for  miles  as  he  rode  along,  and  would  run  away 
from  their  own  mothers  when  the  latter  moved  toward  them. 
He  explained  this  as  adapted  to  the  fact  that  ordinarily 
their  first  duty,  on  making  their  appearance  in  the  world, 
is  to  keep  up  with  the  receding  herd,  while  an  approaching 
object  is  more  likely  to  be  an  enemy  (188).  Later,  this 
rough  adjustment  is  modified;  they  learn  by  experience 
not  to  run  away  from  their  mothers,  and  not  to  follow  indis- 
criminately any  leader. 

If  it  is  true  that  instinct  unmodified  by  experience  is 


292  The  Animal  Mind 

adapted  to  general  rather  than  to  special  features  of  environ- 
ment, it  seems  likely  that  the  phenomena  of  attention  as  we 
know  them  are  found  chiefly  in  connection  with  those  re- 
sponses to  vitally  important  stimulation  which  are  determined, 
in  part,  at  least,  by  the  individual  experience  of  the  reacting 
animal,  for  these  are  the  responses  requiring  most  careful 
discrimination  among  stimuli,  and  the  delay  of  reaction  until 
such  discrimination  has  been  made.1  Putting  the  matter 
in  a  slightly  different  way,  we  may  say  that  purely  inherited 
responses  can  be  adapted  only  to  certain  broad,  roughly 
distinguished  classes  of  stimuli,  for  these  alone  are  common 
to  the  experience  of  all  members  of  the  species.  Nothing 
but  individual  experience  can  bring  to  light  the  importance 
for  welfare  of  certain  particular  stimuli,  for  the  significance 
of  these  would  vary  with  the  experience  of  each  individual 
animal.  Among  the  lower  animals,  attention  probably 
reaches  its  highest  pitch  where  the  response  most  needs  to  be 
suspended  in  order  that  the  stimulus  may  be  fully  discrim- 
inated. The  rabbit  or  wild  bird  crouching  motionless  close 
to  the  ground,  watching  each  movement  of  a  possible  enemy, 
suggests  strongly  to  our  minds  a  condition  of  breathless 
attention.  Whether  such  an  interpretation  is  the  true  one 
depends  very  much,  I  should  say,  on  the  extent  to  which 
past  individual  experience  has  refined  the  animal's  powers  of 
discrimination.  Mere  "freezing  to  the  spot"  may  be  an 
inherited  reaction,  useful  in  time  of  danger,  but  more  anal- 


1  In  this  connection  Franz's  recent  experimental  demonstration  that  the 
frontal  lobes,  long  regarded  as  the  seat  of  the  neural  processes  underlying 
attention,  are  concerned  in  the  functioning  of  recently  learned  reactions,  is 
of  especial  interest.  Franz  found  that  cats  and  monkeys  which  had  been 
trained  to  work  mechanisms  lost  the  power  to  do  so  when  the  frontal  lobes 
were  extirpated,  although  habits  of  older  date,  such  as  responding  to  a  call, 
were  preserved  (136,  136  a). 


Some  Aspects  of  Attention  293 

ogous  in  its  psychic  aspect  to  the  blank  emptiness  of  the 
hypnotic  trance  than  to  alert,  watchful  attention. 

Yet  although,  in  so  far  as  attention  is  a  state  favoring 
discrimination  of  stimuli,  it  is  involved  in  that  part  of  an 
animal's  behavior  which  is  derived  from  individual  expe- 
rience, since  pure  instinct  discriminates  but  roughly;  in  so 
far  as  it  is  still  one  of  the  devices  for  securing  reaction  to 
stimuli  of  vital  importance,  its  root  must  lie  in  instinct.  No 
object  wholly  unrelated  to  some  fundamental  instinct  can 
hope  to  secure  attention,  for  the  great  classes  of  vitally 
important  stimuli  have  all  of  them  preformed  paths  in  the 
nervous  system  by  which  their  reactions  are  secured.  What 
individual  experience  does  is  to  refine  upon  the  adaptations 
which  instinct  makes  possible ;  to  bring  about  the  connection 
of  certain  stimuli,  originally  indifferent,  with  the  performance 
of  an  instinctive  response,  or  to  produce  a  checking  of  the 
instinctive  response  when  certain  individual  peculiarities 
of  a  stimulus  that  would  otherwise  call  it  forth  become 
evident.  For  instance,  an  animal  learns  by  experience  to 
come  at  the  call  of  a  human  being  who  feeds  it ;  the  sound, 
originally  without  effect  on  its  reactions,  has  come  to  be 
connected  with  the  nervous  mechanism  of  an  instinct.  The 
chick  pecking  at  small  objects  on  the  ground  learns  by  ex- 
perience to  inhibit  this  instinctive  response  with  reference 
to  objects  having  certain  peculiarities  originally  undiscrimi- 
nated, but  now  in  some  way  emphasized  through  painful 
circumstances  accompanying  his  previous  encounter  with 
them. 

The  most  fundamental  characteristic  of  attention,  then, 
is  perhaps  that  aspect  of  it  which  has  been  called  abstrac- 
tion, the  diminished  effectiveness  of  stimuli  not  attended  to. 
By  virtue  of  this  aspect  we  recognize  that  attention  belongs 
with  instinct  as  being  concerned  in  securing  the  prepotency 


294  The  Animal  Mind 

of  vitally  important  stimulation.  On  the  other  hand,  the 
further  characteristic  of  attention,  namely,  that  it  is  a  state 
of  suspended  reaction  involving  careful  discrimination  of 
stimuli,  suggests  that  its  functioning  is  connected  rather  with 
the  refining  and  modifying  influence  of  individual  experience 
acting  on  instinct,  since  here  alone  do  we  find  delayed  re- 
action and  accurate  stimulus  discrimination. 

The  highest  grade  of  attention,  the  final  triumph  of  vital 
importance  over  mere  intensity  of  stimulation,  is  to  be  found 
where  the  focus  of  attention  is  occupied  by  an  idea  or  train 
of  ideas.  When  a  process  purely  centrally  excited  holds  the 
field  and  makes  the  individual  deaf  and  blind  to  powerful 
external  stimuli  pouring  in  upon  his  sense  organs,  then  he 
is  superior  to  the  immediate  environment  at  least.  This 
form  of  attention  occurs,  probably,  only  when  the  vital  im- 
portance of  the  idea  attended  to  has  been  learned  through 
that  most  rapid  form  of  individual  acquisition  of  experience 
which  involves  the  revival  of  the  past  in  idea.  It  has  been 
called  derived  attention.  The  ideas  attended  to  are  held 
in  the  focus  of  consciousness  and  analyzed  through  the  power 
of  associated  ideas.  The  inventor  holds  to  his  problem,  the 
student  to  his  task,  in  spite  of  distractions,  because  of  the 
consequences  which  he  thinks  of  as  likely  to  result.  It 
seems  unlikely  that  attention  in  this  final  form  occurs  among 
the  lower  animals.  While  ideas  are  probably  present  to 
some  extent  in  the  minds  of  the  higher  mammals,  they  are 
hardly  so  far  freed  from  connection  with  external  stimuli  that 
the  animal  can  shut  out  the  world  of  sense  from  its  conscious- 
ness and  dwell  in  a  world  of  ideas. 


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320  The  Animal  Mind 

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INDEX 


Accommodation  of  lens,  201. 

Acephala,  chemical  sense,  80  f.;  hear- 
ing, 107;  vision,  130. 

Actinia,  71,  157,  169,  181,  212  f. 

Actinians,  chemical  sense,  69  ff.;  reac- 
tions to  light,  123;  geotropism,  157; 
learning,  211,  212  f. 

Adamsia,  69  ff.,  212. 

Adaptation,  45,  56,  210  f. 

Affective  qualities,  in  mind  of  Amceba, 
43.  See  Pleasure  and  Unpleasant- 
ness. 

Aiptasia,  70  f.,  190,  212. 

Alburnus,  116. 

Allolobophora,  78  f.,  128,  171. 

Amoeba,  38  ff.,  121,  149. 

Amphibia,  chemical  sense,  102;  hear- 
ing, 117  f.;  vision,  141  f.;  learning, 
222,  231. 

Amphioxus,  101  f.,  139  f. 

Amphipods,  83,  175. 

Amphithoe,  83. 

Anecdote,  method  of,  4  ff. 

Anemotropism,  57,  154,  185,  188  f., 
288. 

Annelids,  chemical  sense,  78  ff.;  hear- 
ing, 107;  vision,  126  f.,  172;  geo- 
tropism, 159  ff.;  learning,  209  f. 

Anticipatory    function    of    ideas,    256, 

273- 

Ants,  taste,  86;  food-finding,  88  f.; 
nest-finding,  90  f.;  nestmates,  94  ff.; 
hearing,  112  f.;  vision,  90,  93,  198; 
mimicry,  198;  learning,  209,  211, 
221,  227. 

Aphaenogaster,  94. 

Arachnids,  chemical  sense,  84  ff . ;  hear- 
ing, 109  f.;  vision,  135  f.;  learning, 

2O8,   2IO. 

Arbacia,  287. 
Argiope,  199. 
Association,  274  f. 
Associative  memory,  30,  205. 
Astacus,  83,  109. 


Attention,    49,    241,     245,     260,    282, 

285  ff. 
Attidae,  84. 
Attus,  252. 

Background,  influence  of,  in  light 
reactions,  182. 

Balanus,  133,  134,  176,  178- 

Bees,  chemical  sense,  97  ff.;  hearing, 
113  f.;  vision,  137  ff.,  196  f.;  learn- 
ing, 257,  260  ff. 

Beetles,  86,  in. 

Behavior,  as  evidence  of  discrimina- 
tion, 60  ff. 

Bembex,  263  ff. 

Beroe,  74  f. 

Binocular  vision,  194. 

Bipalium,  170,  195. 

Birds,  chemical  sense,  102  f.;  hearing, 
119;  vision,  142,  143  f.;  learning, 
119,  143,  197,  223,  227,  233,  257,  268. 

Bispira,  208. 

Blatta,  in. 

Bombyx,  88. 

Branchipus,  133,  162. 

Brightness,  in  relation  to  color  vision, 
i2i,  129,  133  f.,  141,  144.  145- 

Canary,  119. 

Carcinus,  83,  219  f. 

Carmarina,  73. 

Cat,  103,  165,  235  f.,  239,  271,  272, 

289,  290,  292. 
Caterpillars,  192,  196. 
Catfish,  214. 
Cebus,  197,  251. 
Centrifugal   force,    orientation   to,    54, 

152,  185. 

Centrostephanus,  131,  208. 
Cephalopods,  130,  160. 
Cerianthus,  70,  157. 
Chemical     sense,     Amceba,     40,     42; 

cceTenterates,  67  ff.;  flatworms,  75  ff.; 

annelids,    78   ff.;     mollusks,    80   f.; 


325 


326 


Index 


echinoderms,  81;  Crustacea,  82  S. ; 
insects,  84  ff.;  vertebrates,  101  flf. 

Chemicals,  effect  on  phototropism,  178. 

Chemotropism,  40,  51,  52,  57. 

Chick,  103,  143,  223,  232  f.,  238,  247, 
257,  293. 

Chlamydomonas,  154. 

Choice,  as  evidence  of  mind,  28. 

Chordotonal  organs,  in  f. 

Chromotropism,  57,  129. 

Cirripedia,  133. 

Clepsine,  128  f. 

Cockroach,  in. 

Coelenterates,  structure,  67;  chemical 
sense,  67  ff.;  hearing,  106  f.; 
reactions  to  light,  123  f.,  169  ff.; 
geotropism,  156  ff.;  localized  re- 
actions, 190;  learning,  208,  209, 
211  ff.,  215. 

Color,  121,  127,  128,  129,  133,  134, 
135,  136,  137,  138,  140,  141,  i43» 
144,  145- 

Convergence,  201. 

Convoluta,  159,  176,  183,  287. 

Copepods,  84,  162  f.,  177,  183. 

Corixa,  in. 

Corymorpha,  69,  157. 

Cow,  271. 

Cowbird,  143,  197,  223,  233,  257. 

Crab,  83,  219  f.,  247. 

Crayfish,  83,  109,  135,  161,  162,  192, 
220  f.,  227. 

Crustacea,  chemical  sense,  82  ff. ;  hear- 
ing, 108;  vision,  132  ff. ;  geotropism, 
161;  learning,  219  f. 

Ctenophors,  74. 

Cyclostomes,  115. 

Cypridopsis,  178. 

Cypris,  179. 

Daphnia,   10,   133  ff.,   172,   174,   i75> 

176  f.,  178. 

Density  of  water,  effect  on  phototaxis, 

177  f.;   on  geotaxis,  156,  161. 
Dermatoptic  sensations,  120  f.,  126  ff., 

130,  136,  137,  139  f.,  142,  184. 
Dias,  134. 
Difficulties  of  comparative  psychology, 

i  ff. 

Dinetus,  101. 

Diptera,  172,  173,  178,  195. 
Direction    theory    of    phototaxis,    167, 

173  ff- 


Discrimination,    evidence    of,    58    ff.; 

method    of,    251    ff.;     in    attention, 

291  ff. 
Distance,  perception  of,   196,   198  ff.; 

effect  of  stimulation  from  a,  275  ff. 
Dog,  25,  103  ff.,  165,  197,  235  f.,  239, 

243,  245  f.,  270. 
Dogfish,  1 1 6,  164. 
Dreaming  in  animals,  15,  271. 
Duration  of  light  stimulation,  effect  on 

phototaxis,  176. 
Dytiscus,  86. 

Ear,  in  fish,  114  f.,  163  ff. ;  in  amphibia, 
117;  in  reptiles,  119;  in  birds,  119. 

Earthworm,  78  f.,  107,  126  ff.,  150  f., 
195,  207,  287. 

Echinoderms,  chemical  sense,  81; 
hearing,  107;  vision,  131;  geotro- 
pism, 160  f.;  learning,  208,  215  f. 

Electrotaxis,  54,  57,  152,  185. 

Eledone,  160. 

Elephant,  239,  243. 

Eloactis,  123. 

Epeira,  199,  202. 

Euglena,  122,  154. 

Evolutionary  writers,  attitude  toward 
animal  mind,  15  f. 

Experiment,  method  of,  9  ff. 

Extirpation  of  sense  organs,  62. 

Eyes,  in  protozoa,  122;  in  coelenterates, 
125;  in  planarians,  .1^6;  in  annelids, 
126,  128;  in  mollusks,  130;  in 
echinoderms,  131;  in  Crustacea,  132; 
in  spiders,  135;  in  insects,  136;  in 
amphioxus,  140;  in  vertebrates,  139, 
142  f.;  in  phototaxis,  174  f.;  in 
balancing,  161,  165;  in  spatially  de- 
termined reactions,  192,  193  ff. 

Fatigue,  63,  65,  72,  167,  210  f.,  213. 

Fish,  chemical  sense,  102;  hearing, 
115  f. ;  vision,  140  f.;  space  per- 
ception, 186  ff. ;  equilibrium,  163  f.; 
learning,  214,  221,  248, 251  f.,  257,  259. 

Flagellata,  57,  122. 

Flowers,  attraction  of  insects  to,  97  f., 
138-  ^ 

Form  discrimination,  196  ff. 

Formica,  93. 

Fovea,  200  f. 

Frequency,  effect  of,  266  ff. 

Frog,  118  f.,  141  f.,  195,  222,  227,  229. 


Index 


327 


Frontal  lobes,  function  of,  292. 
Fundulus,  115  f.,  187,  221. 

Gasteropods,  chemical  sense,  81;  vision, 

130;   learning,  215. 
Gelasimus,  109,  161. 
Geotropism,  57;  in  protozoa,  53,  154  ff., 

286;     in    ccelenterates,    156    ff.;     in 

planarians,  158  f.;  in  annelids,  159; 

in  mollusks,  160;  in  echinoderms,  160 

f . ;  in  Crustacea,  161  ff . ;  in  spiders  and 

insects,  163;    in  vertebrates,  163  ff. ; 

influence  on  phototropism,  162, 182  ff., 

287. 

Gobius,  248,  265. 
Goldfish,  115,  164  f. 
Gonionemus,   73   f.,    124  f.,   150,    151, 

158,  169,  190. 
Grasping   organ,   significance  of,    151, 

280. 

Grasshoppers,  in. 
Guinea  pig,  224,  227,  228. 

Habit,  244,  274  f. 

Hearing,  in  protozoa,  106;  in  ccelen- 
terates, 107;  in  planarians,  107;  in 
earthworm,  107;  in  mollusks,  107; 
in  echinoderms,  107;  in  Crustacea, 
108  f. ;  in  spiders,  109  f. ;  in  insects, 
no;  in  fish,  115  f.;  in  amphibia, 
118  f. ;  in  birds,  119;  in  mammals, 
119;  as  clew  in  maze,  227  f. 

Hedista,  181. 

Heliotropism,   166.    See  Phototropism. 

Helix,  81. 

Hermit  crab,  82,  248  f.,  259. 

Hippolyte,  182. 

Hovering  of  insects,  186,  188. 

Homing,  in  ants,  90  ff.,  in  bees,  98  ff ., 
260  ff.;  in  wasps,  101;  in  birds,  265. 

Hunger,  effect  on  phototaxis,  178;  on 
food-taking,  72,  81,  211  f.;  on  dis- 
crimination, 234. 

Hydra,  67,  123,  157,  169,  208,  209,  210, 

215,  216. 

Hydratation,  181,  194. 
Hydroides,  209  f. 

Ideas,  absence  in  Amoeba,  44  ff.;  learn- 
ing by,  268  f . ;  evidence  for,  in  animals, 

216,  225  f.,  237  ff.,  249  f.,  252  ff.,  270 
ff.;  primitive  function  of,  256,  273  f.; 
relation  to  stimulation  from  a  dis- 


tance, 275  ff.;  of  movement,  279  ff.; 
attention  to,  294. 

Identity,  sense  of,  48. 

Image,  spatial,  193  ff. 

Imitation,  instinctive,  238;  inferential, 
23 8  ff.,  271;  in  bird  song,  119;  self,  283. 

Infancy,  283  f. 

Inhibition,  method  of,  247  ff. 

Insects,  chemical  sense,  85  ff.;  hearing, 
in  ff.;  vision,  136  ff.;  anemotrop- 
ism,  1 88;  geotropism,  163;  learning, 
221,  249,  260  ff. 

Instinct,  inhibition  of,  247  ff.;  unac- 
companied by  ideas,  271;  relation 
to  attention,  289,  291  ff. 

Intensity  of  light,  influence  on  photo- 
taxis,  176. 

Intensity  theory  of  phototaxis,  167, 
173  ff. 

Interactionism,  18. 

Interference  of  stimuli,  285  ff. 

Interpretation,  methods  of,  13  ff. ; 
schools  of,  17  ff.;  precautions  in, 
24  ff. 

Jaculator  fish,  199. 
Jassa,  178. 

Killifish,  115  f. 

Kinaesthetic  sensations,  65,  229  f. 

Kinetic  effect  of  light,  182. 

Labidocera,  176,  179. 

Labyrinth  method,  219  ff. ;  crab,  219  f. ; 
crayfish,  220  f. ;  ant,  221;  fish,  221 
f.;  tortoise,  222;  frog,  222;  chicks, 
223;  birds,  223;  rat,  219,  223  f.; 
guinea  pig,  224,  227,  228;  monkey, 
225;  dancing  mouse,  224,  229. 

Lamb,  291. 

Landmarks,  use  of,  in  homing,  90,  99, 
139,  260  ff. 

Language,  animal,  3  f.,  113,  118. 

Lasius,  92,  94,  96,  137. 

Lateral-line  organs,  116  f. 

Learning  by  experience,  as  criterion 
of  mind,  30  ff .,  205 ;  forms  of,  205  ff . 

Leech,  78. 

Leuciscus,  116. 

Light,  reactions  to.  See  Phototropism, 
and  Vision. 

Limax,  160,  178,  215. 

Limulus,  85,  173. 


328 


Index 


Lineus,  129. 

Littorina,  179  f.,  183,  194. 

Local  sign,  152. 

Locality  survey,  99,  262  f.,  266. 

Localized  stimulus,  reaction  to,  149  £f. 

Loligo,  183  f. 

Lomechusa,  95. 

Macacus,  196,  198,  225,  236. 

Macromysis,  182. 

Mammals,  chemical  sense,  103  ff.; 
hearing,  119;  vision,  144  ff.;  learn- 
ing, 223  ff.,  233  ff.,  251  ff.,  257  f. 

Marginal  bodies,  107  f. 

Mechanical  stimulation,  reaction  to, 
39  ff.,  50,  53,  65,  82 ;  effect  on  photo- 
taxis,  178  ff. ;  on  reaction  to  temper- 
ature, 286;  on  reaction  to  chemical 
stimuli,  287. 

Medusae,  structure,  73;  reactions, 
73  ff.;  static  function  in,  107;  re- 
sponse to  light,  124  f.;  localized 
reactions,  150,  151. 

Metridium,  71,  72,  123,  212  f. 

Mimicry,  198. 

Mind,  evidence  of,  27  ff. 

Mollusks,  chemical  sense,  80  ff.;  hear- 
ing, 107;  vision,  130;  geotropism, 
160;  learning  by  experience,  208  f., 
215. 

Monism,  19. 

Monkey,  105,  144,  196,  197,  198,  225, 
236,  237,  239,  240,  251,  257,  268, 
292. 

Mouse,  dancing,  145,  197,  224,  229, 
239,  243,  258. 

Movement  ideas,  279  ff. 

Movement  of  sense  organ,  203  f. 

Moving  stimulus,  reaction  to,  74,  149, 
190  ff.,  203,  289. 

Myriapods,  136,  196. 

Myrmica,  95,  96. 

Mysis,  108,  161. 

Nautilus,  193. 
Necturus,  199. 
Nest-finding.     See  Homing. 
Nest-smell,    in   ants,   94  ff.;    in   bees, 
99  ff. 

Objective  nomenclature,  21  ff. 
Objects,  perception  of,  279  f. 
Ophiura,  216. 


Orchestia,  176,  177,  179. 
Organic  sensation,  3,  43,  44,  65. 
Orientation,  to  centrifugal  force,  54,  152, 

185;    to    current,    see    Rheotropism; 

to   electric   current,  see  Electrotaxis ; 

to  gravity,  see  Geotropism;  to  light, 

see  Phototaxis. 

Orienting  reactions,  53,  149,  152  ff. 
Orthoptera,  in. 
Otocyst,  106,  107,  157. 
Otolith,  106,  107. 

Pagurus,  84. 

Pain  sensation,  65,  80,  249  f.,  267. 

Palaemon,  108,  161,  162. 

Palaemonetes,  108,  177. 

Parallelism,  17. 

Paramecium,    49    ff.,    150,    156,    186, 

206  f.,  286. 

Past  stimulation,  effect  of,  45  ff.,  56  ff., 

207  ff. 

Pecten,  130,  208. 
Penseus,  161. 
Perch,  248. 
Perichaeta,  171. 

Periodical  fluctuations,   in  geotropism, 

159;    in  phototropism,  179  ff. 
Permanence  of  learning,  267  f. 
Photopathy,  166  ff. 
Phototaxis,  57,  166  ff. 
Phototropism,  57,  122  ff. 
Physiological    condition,   effect    of,   on 

reactions,   64,    68,    71,    77,    81,    135, 

175  ff.,  207. 
Physiologists,  attitude  of,  toward  animal 

mind,  16  ff. 
Pigeon,   143  f-,   197,  223,   227  f.,  233, 

257;  carrier,  265. 
Pike,  248. 
Pitch   discrimination,    in   insects,    in; 

in  birds  and  mammals,  119. 
Planarians,  structure,  75;    contact  and 

food  reactions,  76  ff.;  hearing,   107, 

vision,      126;      localized      reactions, 

150    f.;     righting    reaction,    158    f.; 

photopathy,  167,  170;    learning,  217. 
Platyonichus,  161. 
Play  of  animals,  282  f. 
Pleasure,  as  accompaniment  of  positive 

reaction,  43;   as  motive,  243,  247  ff., 

258,  260. 
Pomace  fly,  182. 
Porthesia,  178. 


Index 


329 


Preference  method,  60,  133,  135,  137, 

140. 
Prepotency  of  vitally  important  stimuli, 

288  ff. 
Protozoa,      structure      and      behavior, 

37  ff.,  49  ff.;    mind,  41  ff.,   54  ff.; 

reactions    to    light,    121    f.,    168    f.; 

geotropism,    154   ff.;    learning,    208, 

214. 
Putting  through  a  movement,  effect  on 

learning,  241,  271. 
Puzzle-box    method,     232    ff.;      birds, 

232  f.;   rats,  233  f.;   cats  and  dogs, 

235;  monkeys,  236;  raccoons,  236  f. ; 

general  character,  232,  245,  260. 
Pycnogonids,   135. 

Raccoon,  119,  144,  196,  197,  236  f., 

242  f.,  252  ff.,  257,  272. 
Ranatra,  179,  249,  287. 
Rat,  219,  223  f.,  226  ff.,  233  f.,  239, 

240  f.,  267. 

Reaction  time,  as  test  of  sensory  dis- 
crimination, 63  f.,  80,  118. 
Recency,  effect  of,  266  ff. 
Repeated  stimulation,  effect  of,  207  ff., 

266  ff. 
Reptiles,  chemical  sense,  102;  vision,  119, 

142,  199  f.;   " sense  of  support,"  165, 

199 ;>  learning,  222  f. 
Rheotropism,  53,  57,  154,  183  ff.,  185. 
Righting     reactions,     153,     157,     158, 

162. 

Sagartia,  72,  123. 

Salamander,  142,  199. 

Sarsia,  124. 

Saturnia,  87. 

Scardinius,  164. 

Sea-horse,  164.  , 

Sea-urchin,  Si,  131,  160  f.,  2o8,_28j. 

Self -imitation,  283. 

Semicircular  canals,  115. 

Semotilus,  140  f.,  251  f.,  257,  259. 

Sense    organs,    significance   of,    59   ff., 

288  f. 

Sex  reactions,  84,  87  f. 
Shadows,   reaction  to,    125,    128,    130, 

Shark,  163. 
Silkworm,  87. 
Simocephalus,  134,  172. 
Size  discriminations,  194  ff. 


Skioptic  reactions.     See  Shadows. 
Smell,  in  annelids,  78  f.;   in  mollusks, 

81;     in   echinoderms,    81;     in   crus- 

tacea,  84;   in  spiders,  84;   in  insects, 

86   ff.;    in  vertebrates,    101   ff.;    as 

guide  in  labyrinth,  226  f. 
Snail,  81,  130. 
Space  perception,   148,   152,   154,   166, 

185,  186  ff.,  191,  192,  193  ff.,  202  ff. 
Sparrow,  English,  119,  143,  197,  223, 

233»  257'»  vesper,  223. 
Spatially  determined  reactions,  148  ff. 
Speed   of   reaction,   influence   on   con- 
sciousness, 202. 
Spiders,   chemical  sense,  84;    hearing, 

109    f. ;    vision,  135   f.,  199,  201   f. ; 

learning,  6  f.,  208,  210. 
Spirographis,  172. 
Starfish,  81,  131,  160,  215. 
Statocyst,   107,   108,   109,   153,   157  f., 

159,  160,  161  f.,  163,  165  f.,  216. 
Statolith,  107,  108,  109,  153. 
Stenamma,  95,  96,  221. 
Stentor,  208,  214,  216. 
Stereoscopic  vision,  200  f. 
Stickleback,  187  f. 
Stoiachactis,  72. 
Structure,  as  evidence  of  mind,  34  ff.; 

as  evidence  of  discrimination,  59  ff. 
"Support,  sense  of,"  165,  199  f. 
Suspended  reaction,  relation  to  space 

perception,   202;    to  ideas,   375   ff.; 

to  attention,  291  ff. 
Swarm  spores,  166,  170. 

Talorchestia,  173. 

Tealia,  71,  214. 

Temora,  178. 

Temperature,  reaction  to,  189 ;  Amoeba, 
39;  Paramecium,  50,  56,  286;  Beroe, 
75;  Amphioxus,  101  f.;  planarians, 
217;  sensation,  65;  influence  on  pho- 
totaxis,  177,  179,  287. 

Tetramorium,  95. 

Thigmotaxis,  52  ff.,  57,  78. 

Tiaropsis,  124. 

Tortoise,  165,  199,  222. 

Touch,  48,  55,  64  ff.,  67,  69,  71,  72  ff., 
76  f.,  78,  82,  84,  86. 

Trial  and  error,  167,  168,  171,  206  f., 
216  ff.,  266. 

Tropism,  16,  18,  57. 

Tubularia,  68  f.,  132. 


330 


Index 


Ultra-violet  rays,  reaction  to,  ito,  127, 
134  f.,  137,  209;  effect  on  photo- 
taxis,  178. 

Uniformity  of  reaction  as  evidence  of 
absence  of  consciousness,  29. 

Unpleasantness,  43,  55,  207,  317,  244  f., 
249  f.,  258,  260. 

Useless  movements,  dropping  off,  219 
ff.;  persistence  of,  231. 

Vanessa,  181,  184,  195. 

Vertebrates,  chemical  sense,  101  ff.; 
hearing,  114  ff.;  vision,  139  ff.; 
reactions  to  gravity,  163  ff.;  learn- 
ing, 214,  221  ff.,  232  ff.,  248  f., 
251  ff. 

Virbius,  109. 


Vision,  in  protozoa,  121  ff.;    in  ccelen- 
terates,   123  ff.;    in  planarians,   126; 
in  annelids,  126  ff.;   in  mollusks,  130; 
in  echinoderms,  131  f.;   in  Crustacea, 
132  ff.;    in  spiders,  135  f.;   in  myria- 
pods,    136;    in   insects,    136   ff.;    in 
Amphioxus,    139  f.;  in  fish,    140  f. 
in  amphibia,  141  f.;    in  reptiles,  142 
in  birds,  143  f.;  in  mammals,  144  ff. 
use  in  homing,  90,  93,  99,  139,  260  ff. 
in  rheotropism,  183  ff.;    in  labyrinth, 
227  f.;   binocular,  200  ff. 

Volvox,  169. 

Vorticella,  208. 


Wasps,    social,    101,    257,    262, 
solitary,  101,  139,  197,  262  ff. 


271; 


INDEX   OF   NAMES 


The  numbers  refer  to  the  pages  on  which  the  work  of  the  writers  is  cited,  whether  their 
names  appear  in  the  text  or  not. 


Adams,  128. 
Aderhold,  154. 
Allabach,  72,  213. 
Allen,  224,  227,  228. 
Andreae,  98. 
Arkin,  128,  171. 
Axenfeld,  184. 
Ayer,  158. 

Baldwin,  283. 

Bardeen,  76,  126. 

Bateson,  83,  107,  133,  140. 

Beer,  17,  21,  108,  161,  201. 

Bell,  83,  109,  135,  192. 

Bentley,  140,  251  f.,  257,  273. 

Berry,  240  f. 

Bert,  134. 

Bertkau,  85. 

Bcthe,  10,  n,  17,  18,  20  f.,  83,  90  ff., 

95  f.,  98  f.,  107,  109,  161,  163, 164, 

247  f.,  261. 
Bigelow,  115,^164  f. 
Bohn,  82,  130,  157,  159,  167,  169,  173, 

175,  180  f.,  182,  194. 
Bouvier,  265. 
Boys,  1 10. 
Breuer,  163. 
Bunting,  161. 
Burnett,  126,  170. 
Buttel-Reepen,  von,  98  ff.,  138!.,  261  f. 

Cannon,  135,  172,  174. 

Carpenter,  182. 

Claparede,  17,  18,  23,  265. 

Cole,  L.  J.,  170,  171,  195. 

Cole,  L.  W.,  119,  144,  196,  197,  236  f., 

242  f.,  252  ff.,  257,  271,  272. 
Conradi,  119. 
Cyon,  163. 


Dahl,  109,  199,  252. 

Darwin,  8,  15,  16^79,  107,  126,  127, 


287. 
Davenport,  135,  155,  160,  172,  174. 


Dearborn,  162. 
Delage,  109,  161. 
Deflinger,  39. 
Descartes,  13,  14-15. 
Drew,  107. 
Dubois,  130,  131. 
Dufour,  137. 

Edinger,  249. 
Eigenmann,  140. 
Emery,  112. 
Engelmann,  106,  122. 
Enteman,  262,  271. 
Esterly,  162  f.,  183. 

Fabre,  88,  262,  264  f. 

Ferton,  265. 

Fielde,  91  f.,  209,  221. 

Fiske,  284. 

Flechsig,  224. 

Fleure,  72,  213. 

Flourens,  163. 

Forel,  17,  19,  86,  87,  92,  94,  96,  97,  113, 

137,  183  f.,  196  f.,  203,  257. 
Frandsen,  160,  178,  215. 
Franz,  292. 
Frohlich,  160,  161,  164. 

Gamble,  159,  176,  182,  183,  287. 

Garrey,  187  f. 

Ghinst,  von  der,  157. 

Giltay,  98. 

Glaser,  216. 

Goldsmith,  248,  265. 

Goltz,  163. 

Graber,  10,  60,  86  f.,  in,  112,  127,  140. 

Groom,  176. 

Groos,  282. 

Gurley,  187. 

Hargitt,  123,  129,  209  f.,  an. 
Harper,  171. 


Harrington, 
Hensen,  108,  109. 

331 


332 


Index  of  Names 


Herrick,  102,  214. 

Hertel,  127. 

Hesse,  122,  126,  128,  193,  202,  208. 

Hobhouse,  239  f.,  243,  245  f. 

Hoffmeister,  126,  287. 

Holmes,   83,   122,   171,   173,   174,   175, 

176,  177,  178,  179,  249,  287. 
Holt,  174. 
Hudson,  291. 
Huggins,  220. 
Hume,  i. 

James,  280. 

Janet,  112. 

Jennings,  39  ff.,  45  ff.,  50  ff.,  62,  64,  121, 
123,  150,  156,  168,  170,  183,  186,  206, 
207,  208,  210,  211,  212,  214,  217, 
244,  285  ff. 

Jensen,  155. 

Jordan,  17,  23,  24. 

Joubert,  133. 

Judd,  275. 

Keeble,  159,  176,  182,  183,  287. 

Kellogg,  87. 

Kienitz-Gerloff,    98. 

Kinnaman,    105,    144,    196,    198,    225, 

236»  239,  240,  257. 
Kline,  12. 
Koranyi,  142. 
Korner,  115. 
Krause,  140. 
Kreidl,  115,  162. 

Learning,  121. 

Lee,  115,  164,  174. 

Locke,  3. 

Loeb,    10,    16,    17,   30,   31,   34,   70,   72, 

137,  157,    163,    164,    166,    167,    172, 
173,  176,  177,  178,  183. 

Lubbock,  10,  88,  89,  90,  92,  133,  137, 

138,  194,  197,  260. 
Lukas,  34  f. 

Lyon,  155,  156,  161,  177,  i86f. 

McCook,  85. 
Massart,  1 54  f . 
Mast,  122,  168,  169,  217. 
Mayer,  in. 
Mendelssohn,  189. 
Merejkowsky,  134. 
Metcalf,  80. 
Mills,  ii,  165,235. 


Minkiewicz,  129. 

Mitsukuri,  180. 

Mobius,  248. 

Montaigne,  13  f.,  15. 

Moore,  156. 

Morgan,  7,  n,  24  ff.,  31,  108,  143,  206, 

238,  247,  259,  271. 
Murbach,  107,  158. 

Nagel,  17,  34,  69,  72,  73,  74,  75,  79, 
80,  85,  102,  106,  107,  123,  128,  130, 

139,   158,    190,   193,  208,  209,  212. 

Norman,  80. 
Nuel,  17,  21,  22. 

Oelzelt-Newin,  62. 
Oltmanns,  169. 
Ostwald,  176  ff. 

Parker,  71,  80,  84,  102,  112  f.,  115  ff., 
117,  126,  128,  139  f.,  142,  162,  170, 
171,  176,  177,  179,  181  f.,  184,  195, 

212. 

Patten,  85. 

Pearl,  76  f.,  85,  150,  158  f. 

Pearse,  69,  123. 

Peckham,  5,  6,  7,  84,  101,  no,  135  f., 

199,  208,  262  ff. 
Perkins,  160. 
Perris,  101. 
Pieron,  94,  212,  221. 
Plateau,  97  f.,  136,  138,  184,  192,  195, 

196,  199. 
Pollock,  69. 
Porter,  143,  197,  199,  223,  233,  257, 

267. 

Pouchet,  133,  172,  178. 
Prentiss,  109. 
Preyer,  10,  107,  215,  216. 
Pritchett,  84,  no. 

Radl,  155,  163,.  175,  185,  186,  188. 

Raspail,  103. 

Rawitz,  130,  192,  208. 

Rhumbler,  38  f. 

Riley,  87. 

Romanes,  4,  8,  9,  10,  n,  16,  30,  31,  72, 

81,  103  f.,  107,  160  f.,  199. 
Roubaud,  188,  287. 
Rouse,  143  *-,  197,  223,  227  f.,  231,  233, 

257- 

Royce,  35. 
Ryder,  130. 


Index  of  Names 


333 


Schneider,  265. 

Schwartz,  154. 

Sewall,  163. 

Sherrington,  191,  278. 

Small,    165,   219,   225  ff.,  231,  233  f., 

239,  267. 

Smith,  78,  79,  127. 
Sosnowski,  156. 
Spaulding,  248  ff. 
Steiner,  163. 
Strasburger,  166,  168,  177. 

Thorndike,  8,  n,  12,  16,  197  f.,  219, 
221,  223,  232  f.,  235  f.,  237  ff.,  251, 
268,  271  f. 

Tiedemann,  131. 

Titchener,  23. 

Torelle,  142. 

Torrey,  69,  72,  157. 

Tower,  112. 

Towle,  178  f. 

Triplett,  115,  248. 

Turner,  92  f.,  194. 

Uexkiill,  von,  17,  21,  131,  208,  287. 

Vaschide,  12. 

Verworn,  n,  107,  154,  158,  166,  174. 


Wagner,  68,  157,  208,  215. 

Walton,  73,  213. 

Wasmann,    17,    18   f.,    22,    90  ff.,  95, 

113,  198,  211. 
Watson,    194,     223    f.,    227,    228   ff., 

234  f. 
Weld,  113. 
We"ry,  138. 
Wheeler,  188. 
Whitman,  128  f.,  199. 
Will,  86,  112. 
Willem,  130,  192. 
Wilson,  123,  169. 
Woodworth,  280. 
Wundt,  5  ff. 

Yerkes,  A.  W.,  210. 

Yerkes,  R.  M.,  34,  35,  64,  73  f.,  118  f., 
134,  141  f.,  145  ff-,  151.  158,  165, 
169  f.,  172,  176,  177,  179,  189,  190, 

197,    199    f.,    220   f.,    222    f.,    224,    227, 

229,  239,  257  f.          ) 

Yung,  8 1,  130. 

Zenneck,  116. 
Ziegler,  17,  21,  22. 


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